Inherited sterility in Lepidoptera.

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INHERITED STERILITY

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Annu. Rev. Entomol. 1975.20:167-182. Downloaded from www.annualreviews.org by Cornell University on 07/09/12. For personal use only.

IN LEPIDOPTERA David T. North Metabolism and Radiation Research Laboratory, Agricultural Research Service, U.S. Department of Agriculture, Fargo, North Dakota 58 1 02

Lepidoptera include some of the most destructive insect pests of agriculture. In the past decade, the increasing resistance to insecticides, along with the more restrictive use of many chemical pesticides, has prompted research for new avenues of insect control. Many researchers have studied the possibilities of population suppression through the release of radiation-sterilized moths, trying to emulate the success with the screwworm fly (Cochliomyia hominivorax) (9). These attempts were not re sounding successes, though there have been several exceptions. Through the release of sterile moths, control of the codling moth (Laspeyresia pomonella) (72, 73) was obtained in isolated orchard tests, and the tobacco hornworm (Man duca sexta) (85) was controlled on the island of St. Croix, U.S. Virgin Islands. Both these successes were under somewhat isolated conditions and on a relatively small scale. Lepidoptera require large doses of radiation to effect sterility compared with most other insects and have often been referred to as being radioresistant (43, 58). Moths are radioresistant when the criterion is induced dominant lethality or sterility. However, they are equally as radiosensitive as the house fly (Musca domestica) (60) when lifespan is the criterion. When Lepidoptera are given sterilizing doses of radiation, induced physiological disturbances (36, 60, 63, 64) are manifested, such as lack of or insufficient sperm transfer, lack of mating, etc. In fact, radiation-induced sterility in a majority of the lepidopteran species investigated is not due primarily to dominant lethality but is more directly related to unfertilized eggs. It was the realization that these debilitat ing effects would limit the ability of the sterile males to compete with natural males in suppressing a population that prompted the reinvestigation of inherited sterility (58, 60, 101). It was believed that more competitive insects would result from using partially sterilizing doses of radiation. Proverbs (71), using the codling moth, was the first to observe that the progeny of irradiated males were semi- to completely sterile. The surviving progeny of irradiated parents inherit sufficient genetic material to make them partially or completely sterile; this is not in itself unique (84). It is important, however, that in 1 67


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contrast with other orders, the progeny of irradiated parents are more sterile than their parents. Chromosome translocations were attributed to be the basis for the inherited sterility in Lepidoptera (6 1). Although the Japanese had used genetic markers to identify translocations (95) in genetic studies with the silkworm (Bombyx mori), Bauer (7) was the first to describe the cytogenetic behavior of translocations in Lepidoptera. There are some unique advantages of using such a phenomenon in controlling or suppressing native insect populations (42). This chapter will deal with what is generally known regarding the efficiency of induction of inherited sterility and the behavior of the F 1 individuals and will then discuss the possible genetic basis of this phenomenon. INDUCED INHERITED STERILITY The first report of induced inherited sterility in Lepidoptera showed that the progeny from codling moth males given 30 krad as pupae were highly sterile, and that the majority of the progeny that became adults were male (71, 74). A summari zation of research on the effects of radiation on the gypsy moth (Porthetria dispar) from 1957 to 1962 (25) showed that: (a) mortality among Fl individuals from irradiated parents (irradiated as pupae) was highest in the larval stage; and (b) moths that had been irradiated as fourth- or ftfth-instar larvae were partially sterile. Results with the Indian meal moth (Plodia interpunctella) and the Angoumois grain moth (Sitotroga cerealella) (13) suggested it was plausible to capitalize on the radiation-induced inherited sterility in these species to suppress later generations. Inherited sterility was then induced in the cabbage looper (Trichoplusia ni) (58) and the sugarcane borer (Diatraea saccharalis) (101), and it has since been induced in over a dozen species (Table I). Not until 1969 (62) was suppression of a laboratory population tried by a single release of partially sterile males. Hence, the salient factors of induced inherited sterility were presented in the years from 1962 to 1968. Increased efforts have been made in the past six years to explore this possibility as a means of insect control.

Methods 0/ Induction Radiation was used in the majority of the studies to induce inherited sterility. There is nothing magical about ionizing radiation; probably any mutagen that is an effec tive chromosome breaker will induce inherited sterility. The general availability of Cobalt-60 and Cesium-137 is responsible for most of the research being done with gamma radiation. Neutrons are more efficient than X or 'Y rays in causing chromo some breaks (47), and therefore, this would hold true for inducing inherited sterility. Neutron sources, not being as available, have not been used extensively but were shown to be more efficient than ionizing radiation in inducing recessive lethals in the silkworm (55) and other insects (49). Inherited sterility is induced more effi ciently by neutrons in cabbage loopers (63, 64) and neutrons enjoy several advan tages over gamma irradiation. First, the percentage hatch of the F{ generation was higher, probably due to a higher degree of fertilized eggs; this means that more

INHERITED STERILITY IN LEPIDOPTERA Table I

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Species of Lepidoptera in which inherited sterility has been induced

Family and species

Crambidae Diatraea saccharalis Galleriidae Galleria mellonella

Referen ce

101 57


Gelechiidae Sitotroga cerea/ella

13

Pectinophora gossypiella

31

Noctuidae Spodoptera exigua

15

Trichoplusia ni

58

Heliothis zea Heliotlzis virescens

63 66

Olethreutidae Laspeyresia pomonella

71

Phycitidae Eplzestia cautella Plodia interpunctella Anagasta kuhniella

1,30 13

H. C. Hofmann and J. G. Riemann, un published observations

Paramyelois transitella

39

Pieridae Pieris brassicae

7

Fl individuals could be placed into a native population in a control program. Second, the required dose to the PI generation to induce completely sterile FI was about one-third less for neutrons than for gamma radiation. More competitive moths would be expected by subjecting later-stage pupae or adults to neutron irradiation. Radiations with a high linear energy transfer (LET) are more effective per unit of absorbed energy in inducing inherited sterility and will most likely give a more competitive PI moth than will gamma irradiation. The immobility of nuclear reactors (the usual source of neutrons) makes this impractical for most programs. There has been very little use of chemosterilants in inducing inherited sterility. Stimmann (86) found that a topical application of tepa in acetone induced both PI and Fl sterility in cabbage loopers. The amount of inherited sterility induced was dose-dependent as it was in irradiation experiments.

Life Stage Irradiated Historically, it has been a common practice of many workers studying radiation induced sterility in insects to irradiate all life stages. The irradiation of early stages (prior to pupa) to induce sterility in species that have monokinetic chromosomes has been inefficient because the induced damage is selected against and not included


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in the newly formed gametes. Species with holokinetic chromosomes (e.g. Lepidop tera) present a different situation since the incidence of cell death would not be expected to be as great. The adult stage, since it is not undergoing metamorphosis, is considered a better radiation target and does not suffer as many of the debilitating effects of radiation (such as malformed wing, improper ecJosion, inability to mate, etc) that plague individuals irradiated earlier (13, 25, 7 1, 75). Adult irradiation, however, does not totally avoid physiological damage (35, 63, 65). The progeny of moths irradiated as either adults or late pupae exhibit the same amount of inherited sterility (13, 15, 57, 102). The dose of radiation required to induce inherited sterility is between 12.5 and 22.5 krad as borne out by studies of the 15 species listed in Table 1. The FI male progeny from irradiated parents in all these species are more sterile than the FI female progeny, regardless of the stage irradiated. The reproductive capability of cabbage loopers irradiated as pupae was equal to that when adults were irradiated only when the dose was fractionated over several days (96). Female sterility was also higher when fractionated doses were given to pupae. Fractionating the dose also is more efficient in Galleria mellonella (100). Pupal irradiation is preferred because the pupal stage is the easiest to handle. Further investigation of dose fractionation of pupal stages would be warranted. Adult codling moths can be chilled before and during irradiation without causing any differences in the amount of F. sterility exhibited by the F. irradiated moths (106). Chilling does not affect the radiosensitivity of the PI generation, and these treatments reduce the injury incurred by restricting the moths' activity. Larval irradiation proved unsatisfactory because the adults resulting from ir radiated larvae usually are not capable of reproduction (2, 15, 19, 20, 25, 76, 102). It was concluded that relatively low doses (3.5 krad) given to fifth-instar larvae of the Indian meal moth induced sufficient genetic damage to spermatogonia to induce sterility (2). The aberrations induced by the low doses are evidently not complex and thus are incorporated into the mature spermatozoa; hence, semisterility is transmit ted by the F. male. Also, the progeny for several generations following irradiation have been reported to be semisterile (37, 57). First-instar G mellonella larvae given 4 krad of gamma radiation developed into partially sterile adult males. This sterility persisted through at least two generations of the male line, and nearly all of the later populations were male (5 7). Oogenesis proceeds later than spermatogenesis in Lepi doptera. It is, therefore, understandable that irradiation of early stages (before fifth instar) would completely destroy the developing germ cells of the ovaries (52) and thus produce infecund females. In Bombyx (95), the percentage of unfertilized eggs decreases with the age of female pupae irradiated. Irradiation of early cell stages of spermatogenesis in Lepidoptera (2, 81, 82, 88, 98, 99) would not be expected to yield genetic damage that would be inherited in any sufficient amount I'equired to be useful in control. Testes containing only gonial cells (35, 83, 89) irradiated with 5 krad had the definitive gonia killed but the predefinitive gonia were more resistant. Consequently, the testes were repopulated with slightly damaged or undamaged


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cells. Researchers exploring the possibility of inducing inherited sterility by irradia tion of larval and embryonic stages must be able to recognize the target germ cells damaged by the radiation and determine their ultimate fate. Induced genetic damage not incorporated into the sperms or ova because of cell death does not lead to inherited sterility. Attempts to induce sufficient genetic damage to yield inherited sterility by ir radiating embryos have generally been futile (6, 13, IS, 19,20,34,39,45,54,71, 76,96, 102). The results were varied because heterogeneous embryonic stages were irradiated. In most cases there was no control over such developmental influences as temperature and photoperiod. As the dose of radiation increases, so does embry onic mortality. The resulting adults, however, are often malformed and fail to mate, but some of the moths that do mate are partially sterile. The female appears to be more radiosensitive when irradiated as an embryo. PI progeny from moths that were irradiated as eggs are usually fertile. The persistence of sterility for several genera tions beyond the PI of irradiated eggs has been reported (38, 57). These exceptions warrant further exploration of embryonic irradiation as a means of inducing inher ited sterility. However,future studies should take into consideration irradiation of precise stages of embryonic development,and control should be exerted over the environment. Temperature and photoperiod during embryonic development play an important role in the subsequent fecundity of codling moths (16). It should be remembered that an embryo, diploid in chromosome complement, is unlike the haploid gamete, and thus the resulting adult would be heterozygous for chromosomal aberrations. This would make the PI of an irradiated embryo the equivalent, genetically, to the FI progeny from an irradiated sperm or ovum. Irradia tion of embryos, if successful, would allow placing irradiated eggs on noneconomic hosts; this would allow rearing sterile moths in nature. This technique could easily be used for the protection of some field crops.

Male-Female Responses As stated earlier, the progeny from irradiated male moths are more sterile than their parents. The progeny of irradiated females exhibit some sterility (62, 63, 68) but are more fertile than their irradiated mothers. Irradiation of female sugarcane borers with as little as 2 krad resulted in the subsequent collapse of the laboratory popUla tion (103). The female progeny of these irradiated females were reported sterile, and the male progeny were semisterile. F 1 males from irradiated female cabbage loopers (15 krad) were more sterile than the PI females (63). However, no difference was found in the sterility between the PI males and females of irradiated tobacco bud worm females at any dose up to 22.5 krad (68). This is inconsistent with the data reported for the sugarcane borer, though it is difficult to visualize what difference in the species could be responsible for giving such a large variance in the dose required and why the PI females are more sterile than PI males. PI tobacco bud worm males from irradiated females did not transfer sperm as well as the untreated males (68). This trait is characteristic of PI males from irradiated male parents (4, 63,66,78).

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The fertility of offspring from irradiated females in most lepidopteran species has an important bearing on the effectiveness of releasing partially sterile females for suppression. This makes it mandatory to release only males if inherited sterility is to be used as a method of suppressing lepidopterous pests. One way to avoid this would be to release both sterile females and partially sterile males. This could be achieved by giving a single dose of radiation to both sexes because females are more radiosensitive than males.


Sex Distortion It has become generally accepted that irradiated male produce more male than female progeny. This was first shown in the codling moth (71) and later in all species examined (15, 30, 39, 57, 62, 66, 100). Contrary to this, irradiated females produce progeny in a normal sex ratio (68). In Lepidoptera, the male is homogametic and the female is heterogametic (29, 51, 79, 95, 105). The male is XX (29) or ZZ (79, 95), and the female is XY or ZW (depending on the nomenclature used for the sex chromosomes). It is also possible that in some species the female would be XO or ZO (87). This is neither the time nor the place to get into a lengthy discussion of the symbolism of sex chromosomes. The reason for using the ZZ:ZW scheme is to point out that the female is heteroga metic (79). Goldschmidt (26-28) beautifully analyzed the sex determination of Lymantria dispar, and I prefer to follow him in retaining the classical genetic sex symbolism of using XX and XY. The portrayal of sex determination as nothing more than the orderly disjunction of a single chromosome is a gross simplification in Lepidoptera. There are many autosomal factors involved (12, 29, 53, 79, 95, 105) in sex determination in Lepi doptera. The reason that irradiated males tend to produce more male progeny than female was suggested as being due to the induction of recessive lethals on the X chromosome of the male (63, 66). Male cabbage loopers fed ethyl methanesulfonate (EMS), an active mutagen but not known as an effective chromosome breaker, produced more male progeny than female (D. T. North, unpublished observations) without sterility. Individual fertile lines were isolated in the F2 generation that produced two males to one female. This, along with the fact that the sex ratio of the progeny of the irradiated females (66) is 1:1 is considered evidence that sex distortion is caused by lethals on the X chromosome. When all the progeny from irradiated females were grouped together, the sex distortion was in favor of males, whereas if individual pairs were analyzed, some lines showed distortion toward an excess of females (66). Catcheside & Lea (10) found sex-ratio distortion in irradiated Drosophila melanogaster and concluded it was caused by damage to sperms bearing the X chromosome. This gave an excess of males in Drosophila, but in Lepidoptera it would reduce the number of XX individuals (males), giving an excess of females. Therefore, it is possible that distortion of both sexes exists in Lepidoptera; however, the majority of the distortion creates an abundance of males. The de termination of the beneficial effect of sex distortion on population suppression and manipulation has not been examined except for a unisexual (18) strain of Esligmene acrea.


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Development of FI Progeny F I progeny of irradiated parents develop more slowly than progeny resulting from a mating of untreated individuals (11, 63, 66), and this phenomenon is dose-depend ent. The lag in dev elopment is also temperature-dependent (67). When progeny were reared at high temperatures (27°C), there was more radiation damage expressed and some progeny appeared to enter diapause. This points out the severe physiological debilitation suffered by these FI individuals. The possible hormone imbalances existing in the progeny of irradiated parents have not been studied, but this incident in the budworm (67) shows that the situation exists. This could be a limiting factor under field conditions for achieving population suppression using inherited sterility. The fact that these individuals would be reared in nature (61) has been presented as an argument for their being hardier than the PI-released, laboratory-reared moths. Certainly the limited data available on development under known conditions would make such a conclusion suspect. Before using inherited sterility as a possible method of suppression, more data are needed on behavior of the F1 moths in a natur al environment. Sperm Production and Transfor The male progeny from males given a partially sterilizing dose of radiation often fail to transfer sperm to the spermathecae of the female successfully (11, 30, 44, 64, 66, 102), but no quantitative data on sperm production and transfer are available. The relative amount and type of sperm transferred to the spermatheca by the FI male was approximated (11, 44) in the pink bollworm (Pectinophora gossypiella). These approximations are crude at best but do serve to illustrate that FI males have a problem in sperm production and transfer. It was observed that FI progeny of males treated with apholate had few eupyrene sperms (89), although they produced a normal number of apyrene sperms. It has not been determined whether fewer

eupyrene sperms are produced or whether eupyrene sperms are unable to reach the spermathecae. It is impossible to differentiate between the two from present data.

Now that there are proven methods to determine the number of sperms in the spermathecae (77), quantitative evaluation of sperm transfer by FI males is possible

and should be studied. Riemann (78) examined the sperm of male FI Mediterranean flour moths, the male parents of which had received 15 krad. Sperms were observed from the spermatheca of an unirradiated female to which the FI male had mated or from sperm bundles in the seminal vesicle. The bundles in the seminal vesicles had reduced numbers of cells, and these cells contained a variety of abnormalities. All of the intact sperms in the spermathecae appeared morphologically normal. Appar ently only morphologically normal cells are maintained in the spermathecae. The FI progeny of Indian meal moths, the male parents of which were irradiated as adults, inherited more ultrastructural abnormalities than those from PI males ir radiated as first-instar larvae (3). It is in teresting to note that the PI males irradiated as fifth-instar larvae exhibited a high frequency of flagellar and apical abnormalities of e up yrene sperms. The damage inflicted on the germ cells of the fifth-instar larvae

is eliminated when the abnormal sperms are lost. It is questionable, however,

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whether the abnormalities found in the Fls from irradiated adults are due to inheri tance of the genetic damage or are simply an imbalance of the system, manifesting itself as abnormal sperm. This point definitely needs clarification. INHERITED STERILITY BY GENETIC MANIPULATION


Hybridization There are numerous reports of hybridization of species and subspecies in Lepidop tera (79), but their value to genetic understanding has been very limited. Since many hybrids are sterile in the first generation, little genetic information can be derived from hybridization. Although hybridization of lepidopterous species has been stud ied for over three quarters of a century, this area was not explored as a source of sterility for population suppression until 1959 (17). Hybridization offers a method of inducing sterility without the debilitating effects of radiation, though hormone imbalance, etc is found in hybrids. A hybrid that has limited fertility affords an opportunity to manipulate the genetic material so a vigorous, but sterile, insect would result. Laster (46) found that the Hetiothis subflexa X Heliothis virescens hybrid could be made in the laboratory. The progeny from such a cross are of interest since the F \ males are sterile and the F \ females are fertile when crossed to H. virescens. Although the F \ female is fertile, her male progeny are sterile. This would appear to be an exception to the rule expressed by Haldane (33) that in hybrids of animals, the rare or sterile sex is the heterogametic sex. There is no reason to suspect, however, that the males in Heliothis species are not homogametic. Proshold & LaChance (69) have expanded upon the original work of Laster and were able to make the reciprocal hybrid crosses between H. subflexa and H. vires cens. The F\ males from both reciprocal crosses were nearly sterile when back crossed to either parent species. The female Fls were semisterile, depending upon the interspecific cross. Over 40% of these F \ females resulting from H. subj/exa X H virescens entered diapause under conditions which normally do not induce diapause in either species. The nondiapausing females were reluctant to mate with males of either parent species. The male progeny were sterile through three back cross generations of these females X H. virescens males. Females from a reciprocal cross, however, produced completely fertile male progeny following three back crosses to male H. virescens. Not all hybrids give sterility (56), but their progeny can inherit chromosomal anomalies. The male sterility reported in Heliothis hybrids (46, 69) was apparently from the lack of fertilization. This conclusion was reached because of the absence of eupyrene sperm in the spermathecae of females mated to hybrid males. Sperm bundles were observed in spermatophores transferred by hybrid males and the eupyrene bundles were described as inactive, usually with scattered sperms (46). This is not necessarily an abnormal situation (36), but failure to recover many eupyrene sperms in a spermatheca would be indicative of problems related to the transport of the eupy rene sperms to the spermathecae. A majority of the eupyrene sperms are morpholog ically abnormal (R. Richard, personal communication) and, therefore, it is possible



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that females eliminate these abnormal sperms rather readily from the spermathecae (78). If these observations of the spermathecae are not made soon after the arrival of the sperm, they would be missed in the analysis. Hybrids afford many types of cytogenetic manipulations to give sterility that can possibly be used for popUlation suppression. One outstanding limitation of using the described Heliothis hybrids would be the possible influx of H. subjlexa genes into H. virescens through the fertile hybrid females, assuming the hybrid females mate in the field with H. virescens males. The adding of genes that could possibly widen host-plant adaptation and possible insecticide resistance to H. virescens could over ride any possible advantage obtained with the sterile males. It would appear that detailed studies are required in both the areas of quantitative genetics and cytogenet ics of these hybrids before considering releasing test insects in the field. With further research, this could prove to be one of the most promising areas where genetic control could be applied to suppression of natural populations.

Translocations Theoretically, population suppression of insects can be achieved by releasing homozygous translocations (84, 104, 107), but its value in controlling Lepidoptera is doubtful. First, lepidopteran species possess large numbers of chromosomes (50) and they lack size and varied morphology that could be used for cytological identifi cation (7, 61, 95). Second, homozygous translocations, in Drosophila at least (8), have proved to be very nonviable. Also, there are no lepidopteran species of eco nomic importance with a sufficient number of mutants to label the chromosomes as has been done in Bombyx (95), where over 250 mutants are known. With this extensive knowledge of linkage groups, translocations can be recognized through segregation of these loci in various crosses, and the stock can be easily maintained. It would take a long time to develop such a system for any one economically important species. Bauer (7) conducted a thorough study of translocation behavior in Pieris brassicae. North & Holt (61) related heterozygous translocations to radia tion-induced inherited sterility. Using the technique of partial castration (40), 24 different heterozygous translocations were examined in the cabbage looper (D. T. North, manuscript in preparation), and there was less than 10% fertility in most cases. Interestingly, there was little decrease in the fecundity of the females to which heterozygous translocation males were mated, inferring adequate sperm transfer. Translocations can be effectively used to sex Lepidoptera (95), though this technique has not been explored for economic species. However, there are enough mutants available in economic species to make this technique worthwhile.

Polyploidy Polyploids have been induced in Bombyx by several methods (95) but have proved impossible to maintain. The tetraploids induced have been females and thus have to be mated with diploid males. This results in triploid offspring. Reports have been made (95) of the establishment of a tetraploid strain from the progeny of a 6N X 2N cross. However, this strain could not be maintained for more than two genera tions.


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Polyploids could be used as a source of sterility in several ways. Sterile triploids can be obtained by crossing 4N X 2N individuals. This would prove impractical because the female line would have to be induced each generation. Triploid individu als could possibly be stockpiled in some species and stored in the diapause condition until needed for release. This would be cumbersome, and the performance of tri ploids would have to be studied first. Also, there is evidence that the sexual repro ductive performance (70) of the moths emerging from diapause is poor. When eggs are centrifugated prior to undergoing first cleavage division, many triploids and aneuploids result (95). There is the possibility that stable aneuploid lines could be established from these individuals which would be intrafertile and produce no progeny when mated to normal individuals. The ability to remove one testis (40) for cytological examination and then test breed the resulting moth can be used in establishing such strains.

THE CYTOGENETIC BASIS OF INHERITED STERILITY The lepidopteran karyotype is characteristically small and has symmetrical chromo somes without much morphological variation. There have been many reports on the holokinetic nature of the lepidopteran chromosome (5, 7, 23, 61, 80, 90-94, 99) and the question is still unsettled (105), but these reports serve as the basis to explain radioresistance and inherited sterility in Lepidoptera (43, 58). The small size and lack of distinct shape has not allowed determination of the type of the kinetochore through chromosomal behavioral studies. There are several methods for preparing lepidopteran chromosomes for cytogenetic examination (21, 32, 56, 61). Transmission electron micrograph studies indicated that lepidopteran chromo somes in gonia and meiotic cells have a localized kinetochore approximately the same size as in monokinetic species (14, 24). During metaphase I they do not appear to have any kinetochore activity (23). A greater number of translocations (61) could be recovered from species with holokinetic chromosomes than in a monokinetic species since all translocations induced would behave as symmetrical exchanges. Thus, the number of recovered translocations from any one dose of radiation would be double that for a monoki netic species. Insufficient data are available to make conclusions regarding the role chromosome translocations play in induced inherited sterility; 100% of the progeny of male corn earworms given 20 krad possessed at least one translocation (59), and when their fathers received only 10 krad, 80% possessed at least one translocation. The biggest problem in assessing the amount of sterility contributed per heterozygous transloca tion is the difficulty of separating fertilized from unfertilized eggs. Eggs laid by females inseminated by FI males are often unfertilized because of the abnormal sperm received from the F I males (4, 78). It is of more than academic interest that we determine cytologically the event or series of events that lead to abnormal sperm production in the Fls. It would appear that the cause may be the same in Fls from irradiated parents as well as for hybrid crosses. An exciting aspect of the hybridization work with Heliothis is the possibility of substituting chromosomes from one species to another. Although 3-7 chromosome


pairs in these hybrids are not found as bivalents at metaphase I

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(69), this is not to

infer lack of homology. First, it is not known whether the lack of pairing is asynaptic or desynaptic in origin. There are many factors that can affect either type of pairing

relationship; i.e. translocations, inversions, etc ( l 05). However, substitution of one or more of the H subflexa chromosomes into the karotype of H virescens with proper selection could yield an intrafertile strain that would be sterile when crossed back to normal H. virescens. It is imperative, therefore, to determine the reason for the lack of pairing observed and the relationship of this to sterility and abnormal


sperm production. Even more important

,

if chromosome substitution lines are

developed this would afford isolation against any intragression of the two species under field conditions.

PRACTICAL APPLICATION OF INHERITED STERILITY FOR POPULATION SUPPRESSION The most obvious drawback to the use of inherited sterility is that the F I progeny need to be reared on a host. (If this host is of economic value, this does not present an appealing method to growers.) There are certain insects that appear more suitable

for control by inherited sterility than others; the sugarcane borer, where cultural practices are such that damage to early tillers does not necessarily hurt the crop; the gypsy moth, where though the Fls would cause some defoliation, there should be sufficient sterility of the PI generation to lower the original infestation, and the Fls could effect suppression during the next generation; the com earworm, particu larly in the southeastern

U.S., because infested com usually contains a single larva

that does little damage and thus Fis would be available to suppress the p opUla tion that infests cotton; and the Asiatic rice borer,

Chilo suppressalis,

where, again,

infestation of the early tillers does not harm the crop value. Protection of the southeastern states from cabbage loopers (42) by releasing partially sterile moths in Florida early in the year, and thus minimizing the populations available for northern

migration, has been suggested. The idea of using partially sterile moths that also transmit conditional lethals

(41) has been suggested. Careful consideration would

have to be given to such a pl an or the two objectives could cancel each other. For maximum effect of inherited sterility, the PI generation should be fertile (22,

62, 102)

and the F I completely sterile. In order to transmit recessive conditional lethals into a population, the FI generation would need to be only semisterile. However, it is a concept worth consideration. Inherited sterility was effective in suppressing cabbage loopers (97) and sugarcane borers (103) in field cages, but there has been no major assessment of the technique under field conditions. Partially sterile cabbage loopers were released on St. George Island, Florida (48), but there was no evidence that the suppression obtained for one month resulted directly from inherited sterility. Cytological analysis of com ear worms collected on St. Croix, U.S. Virgin Islands, during and following the release of male and female moths given 25 krad, showed their progeny were able to survive

(D. T. North and J. W. Snow, manuscript in preparation). As high as 30% of the larvae sampled at one period had chromosome aberrations, indicating that they were the progeny of an irradiated parent. These results provided no data as to the

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efficiency of infusing Fis into a natural population, but clearly indicate that Fls were capable of establishing themselves in a natural population. Based on laboratory studies (62) at least 60% of a population have to be F1s from an irradiated parent for the inherited sterility concept to be effective. The use of inherited sterility is not the complete answer to suppression of all Lepidoptera, but the advantages it affords outweigh the disadvantages for some species. Greater evaluation of the method is needed at the field level before its total worth as a possible control measure can be assessed. AKNOWLEDGMENTS

The author is indebted to Jane Houtkooper and Gail Nelson who kept order in the laboratory while this chapter was in preparation. Special thanks are extended to Diane Kastet for deciphering and typing the manuscript and to Fred Proshold for listening. Heartfelt thanks go to Mrs. Vivian Chaska, who once again survived the incompetence of the author and got the job done. Literature Cited 1. Ahmed, M. S. H., AI-Hakkak, Z., AI

2.

3.

4.

5.

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Tissue sterility in uneviscerated carcasses.

Sterility: an immunologic disorder?

[Sterility due to aberrations].

Sterility testing [proceedings].

Atypical mycobacteria and sterility.

Cytoplasmic male sterility in Brassicaceae crops.

Malarial Influence in Abortion and Sterility.

[Extrakaryotic inheritance of pollen sterility in Oenothera].

Macrophage migration inhibitory factor in female sterility.

Male sterility and fertility restoration in crops.

[Rational hormone diagnosis in normocyclic functional sterility].

[Plastome dependent pollen sterility inOenothera].

Genomic networks of hybrid sterility.

Sterility of blood collection tubes.

Inherited goitre in sheep.

[The difficulties in dealing with sterility].

Sterility of bile in multiple-organ donors.

Causes of Sterility in Bosnia-Herzegovina Population.

Mitochondria and cytoplasmic male sterility in plants.

Levobupivacaine ampoule labelling about sterility.

Sterility problems and the diabetic.

Some Clinical Aspects of Sterility.

Inherited amyloidosis.

Inherited macrothrombocytopenias.