ANNUAL REVIEWS
Further
Quick links to online content Ann. Rev. Genet. 1978. 12:433-50 Copyright © 1978 by Annual Reviews Inc. All rights reserved
ULTRASTRUCTURAL STUDIES
+3145
Annu. Rev. Genet. 1978.12:433-450. Downloaded from www.annualreviews.org by Lomonosov Moscow State University on 11/19/13. For personal use only.
OF CHIASMA DISTRIBUTION Peter B. Moens Department of Biology, York University, 4700 Keele Street, Downsview, Ontario, Canada M3J IP3
CONTENTS INTRODUCTION ........................................................................................................ SC NODES AT CROSSOVER SITES ........................................................................ SC REMNANTS AT CROSSOVER SITES ................................................................
SCs AND THE ACHIASMATIC CONDITION . SC COILING MODELS FOR THE REGULATION OF CHIASMA DISTRIBUTION
......... ............................................
................................................................................................................ ..............
433 436
440 441 443 444
INTRODUCTION
The frequency and distribution of crossovers are precisely regulated in the meiotic prophase nucleus. For a given species the number of crossovers per nucleus deviates little from the average (1), but differences may exist be tween male and female reproductive cells of the same species (2-4). Local races may have slightly different averages (1, 5) and individual differences can be caused by mutations (6--8), B-chromosomes (9-12), chromosome rearrangements (13, 14), amounts of heterochromatin (15-20), or environ mental factors (21-24). Within a given nucleus the crossovers are uniformly rather than ran domly distributed among the bivalents (25). Also within a bivalent the crossovers are not randomly distributed. The phenomenon whereby one crossover reduces the probability of another one occurring nearby is termed crossover position interference (25). The frequency and distribution of cross overs can be measured directly by observation of chiasmata at the diplotene stage of meiotic prophase. It can be measured indirectly from recombinant progeny in the offspring of appropriate crosses. 433
0066-4197178/1215-0433$01.00
Annu. Rev. Genet. 1978.12:433-450. Downloaded from www.annualreviews.org by Lomonosov Moscow State University on 11/19/13. For personal use only.
434
MOENS
In an earlier review of the regulation of crossing-over, Lucchesi &Suzuki (13) discuss a variety of models designed to explain unusual crossover distribution patterns. The models take into account factors such as position effects, bouquet arrangement, centromere repulsion, chromosome coiling, and a competition for some substance, active in crossing-over, which is present in limited quantities in the nucleus and can diffuse along the biva lent. The authors favor the view that the different effects are a function of the timing of meiotic events such as synapsis, crossing-over, and segrega tion. For example, a shortened period of synapsis would reduce the possibil ity of a crossover occurring while a longer period would increase the probability of a crossover taking place. Since the writing of that review, the study of meiotic chromosome behav ior has acquired an additional perspective which originated with the discov ery of the synaptonemal complex (SC) by Moses (26). It has become apparent that meiotic crossing-over invariably occurs in the presence of the SC (27, 28) and models of crossing-over now attempt to integrate the SC in the process (28-32). The expectation is that the structure and behavior of the SC and associated structures may provide a clue as to the timing and distribution of crossing-over along the bivalent at meiotic prophase. The interpretation of the variousSC modifications in relation to crossing over can be based not only on their correlations with chiasma frequency and distribution, but also on their possible function in the process of recombina tion itself. Some of the relevant characteristics of models of recombination are therefore briefly repeated here. Where two homologous chromatids are closely associated, they can become involved in an exchange of DNA strands through one of several mechanisms (30, 33-35). The exchange point can travel along the chroma tids for distances of several hundred base-pairs, thus causing one or both of the chromatids to have an extended region of "hybrid DNA" (25, 29, 35-37). Genetic evidence indicates that these exchanges are formed ran domly but in limited numbers along the length of the bivalent (38). Break age of the exchange DNA strands at this point leaves the parental gene sequences intact with possible postmeiotic segregation or conversion for the region of hybrid DNA. To produce a reciprocal crossover it is necessary to rotate one chromatid 1800 relative to the other one. Following a second 1800 rotation in a plane perpendicular to the first, the nonexchange strands are now the ones that cross over. A cut of these strands produces a reciprocal crossover for outside markers with conversion or postmeiotic segregation for the region of hybrid DNA (35). Such reciprocal crossover events interfere with the occurrence of other reciprocal events nearby, but not with a nearby non reciprocal event (29) (Figure 1). The strand isomeration stage may be a necessary intermediate of reciprocal as well as nonreciprocal events (36, 37),
435
Annu. Rev. Genet. 1978.12:433-450. Downloaded from www.annualreviews.org by Lomonosov Moscow State University on 11/19/13. For personal use only.
CHIASMA DISTRIBUTION
making the first 1800 tum compulsory. A different model has two exchange points and does not require strand isomeration to produce a reciprocal event (30, 39). In his discussion of crossover position interference Holliday (29) has pointed out that models of recombination at the molecular level do not predict or explain interference between crossovers. He postulates that a DNA binding protein is required to stabilize a crossover, possibly in the shape of a recombination nodule (40) (Figure 2). The protein is contained within the SC, and at the site of a crossover the protein is depleted in that region and no crossovers can be formed nearby. A more mechanistic model (41, 42) can be designed on the grounds that purely structural chromosome rearrangements can bring about changes in crossover frequencies and distri bution. The stabilization of an exchange event may take place by way of a reorganization of the lateral elements along the crossover strands. Such a reorganization produces coiled SCs or, where the SCs are coiled prior to lateral element realignment, it can result in a reduction ofSC coiling (Figure 3). In this article I review the various SC modifications that may have a bearing on crossing-over and on position interference.
I ..... �
---II
AX8 ��: )0� .A(
__ A ..... �B
. 1--
C --1 ---"'-- 0
C
7
�
�
0
------,--,1:1--:=::_-===: :::-- ---1-: Figure J
Genetic exchange model (35). Only two of four chromatids are diagrammed. The ' DNA strands or the one chromatid are drawn wide and the strands of the nonsister chromatid are drawn narrow. The nicks are indicated by gaps in the lines. At the sites of the initial nicks an exchange of strands is established. The exchange point
can
travel along the two DNA
strands so that regions of hybrid DNA are formed. The crossed strands can be cut so that the chromatids become separated without signs of recombination between flanking markers, but gene conversion may occur in the hybrid region. Alternatively, the strands can be flipped over so that the nonexchange strands become the crossed strands. A cut now results in a classic crossover event with recombination of flanking markers. Conversion can still occur through excision and repair of mismatches in the hybrid region.
--I
Annu. Rev. Genet. 1978.12:433-450. Downloaded from www.annualreviews.org by Lomonosov Moscow State University on 11/19/13. For personal use only.
436
MOENS
Figure 2a,b
A synaptonemal complex node, N, of the type observed in Drosophila oocytes
and named recombination nodules (40). The node, N, is shown in two consecutive sections of a crane fly spermatocyte. Also shown are a synaptonemal complex Sc. the nucleolus NO, and the lateral elements at the site of the node, arrowheads. The scale bar is 1 p.m.
SC NODES AT CROSSOVER SITES
The most extensive observations of the correlation between chiasmata and SC modifications has been reported by Zickler (43) in the fungus Sordaria macrospora. Light microscopy of cells in diplotene show that there are three to four chiasmata in each of the long bivalents ( 13 and 11 J-Lm). The four medium-sized bivalents (7 J-Lm) have two to three chiasmata, and chromo some No. 7, the shortest one ( 6 /Lm), has one chiasma. The average was 18 chiasmata per nucleus for 20 nuclei scored. At the electron microscope (EM) level, in completely reconstructed pachytene nuclei the SCs contain one to four dense bodies in the central element of the SCs. One nucleus had 17 such SC nodes and another had 21. In two diplotene nuclei theSCs have mostly disappeared but several of the remaining short stretches show SC
437
Annu. Rev. Genet. 1978.12:433-450. Downloaded from www.annualreviews.org by Lomonosov Moscow State University on 11/19/13. For personal use only.
CHIASMA DISTRIBUTION
Figure 3a-f Five consecutive sections of a rat spermatocyte showing a synaptonemal com plex, SC; an SC node, N; the centromeric region of a bivalent, CE; and the nuclear envelope,
NE The node is clearly of a different type from the ones found in diptera (Figure 2) and fungi. It appears to be a bar which connects the lateral elements and lies to one side of the SC. The twists or coils of the SC are shown in the lower SC; all are counterclockwise. The scale bar is 1 p.m.
Annu. Rev. Genet. 1978.12:433-450. Downloaded from www.annualreviews.org by Lomonosov Moscow State University on 11/19/13. For personal use only.
438
MOENS
nodes-18 and 19 respectively. In mouse spermatocytes the SC remnants are at the site of a diplotene chiasma (44, 45) and by extrapolation this may imply that the Sordaria SC nodes are at the site of a crossover. A correlation between SC nodes and crossovers is further suggested by the distribution of the SC nodes. Pooling of bivalents No. 3 to No. 6 inclusive for the three pachytene nuclei gives a total of 30 SC nodes for 12 bivalents. The average is 2.5 per bivalent with a variance of only 0.81, indicating that theSC nodes are uniformly rather than randomly distributed as is expected of chiasmata position interference. Because of the small sample size, however,the distri bution does not differ significantly from a Poisson distribution,p 0.20. Zickler (43) shows that the SC nodes appear before synapsis is completed for all chromosomes. This should not be interpreted that the SC nodes are formed during zygotene. In this and many other fungi the meiocyte is formed by the fusion of two haploid nuclei that are already well into meiotic prophase, pachytene judging by the condensed state of the chromosomes, at the time of fusion (46, 47). The type of SC nodes analyzed by Zickler (43) had previously been reported in two other ascomycetous fungi: Neurospora crassa by Gillies (48) in the yeast Saccharomyces cerevisiae by Byers & Goetsch (49). They have also been reported in Chlamydomonas (50), maize (16), and in Ascaris (51). Unlike tlie Sordaria data, these observations are somewhat sporadic and they do not clarify the correlation betweenSC nodes and crossing-over. If, all the same, there is a causal relationship, the SC nodes by themselves do not provide a clue to the mechanism that regulates their numbers and distribution. Recently the SC nodes in Schizophyllum commune and N. crassa have been described in detail (51a, SIb). In N. crassa Gillies reports an average of 19 recombination nodules per pachytene nucleus (based on 8 nuclei) and the observed number of crossovers from genetic studies is about 18 per nucleus. The average distance between nodules is about 3 p.m but some are as far as 11 p.m apart and in one case 3 nodules were present in a 1 Jl.m stretch of SC. The suspected centromeric region has relatively few nodules, while the distal portions of the SCs have a slight excess. As in Sordaria (43) the nodules appear while chromosomes are still pairing and they stay late, till diplotene, when most of the SCs have disappeared. In S. commune less is known about total chiasmata or crossovers per nucleus so that the nodules cannot be related directly to reciprocal recombi nant events. Their numbers and distribution, however, follow a similar pattern as described for Sordaria (43) and Neurospora (51b). Six nuclei with an average of 29 p.m of SC per nucleus had an average of 22 nodules per nucleus. Nine smaller nuclei with 20 p.m of SC per nucleus had 13 nodules per nucleus. The longer bivalents had one nodule per 2.2 p.m ofSC and the shorter bivalents had one nodule per 1.1 p.m of Sc. =
Annu. Rev. Genet. 1978.12:433-450. Downloaded from www.annualreviews.org by Lomonosov Moscow State University on 11/19/13. For personal use only.
CHIASMA DISTRIBUTION
439
A different type of SC node was reported by Carpenter (Figure 2) (40) in Drosophila melanogaster oocyte nuclei. These are large spheres, three to four times as large as the nodes of Sordaria, located on top of the SC somewhat like a large ball on a ladder. Carpenter named this structure a recombination nodule (RN), to denote its correlation with the frequency and distribution of crossing-over in female Drosophila. In a sequence of early to late pachytene stages, she reports oocyte nuclei with 0, 0, 0, 4, 5, 3, and 4 RNs respectively. The total genetic map length of 9 Drosophila is 280 units, which, when divided by 50, gives an estimate of 5.6 reciprocal crossover events per nucleus. There are 5 arms available so that the expected frequency is 1. 12 crossovers per arm. Accordingly, 2 1 of the 23 recon structed SC arms had one RN each, and two arms had two RNs each. In the latter case they were well separated as predicted by positive crossover position interference. The RNs were found in the euchromatic regions only, which is also in agreement with the observed lack of crossing-over in heterochromatic regions. Carpenter & Baker (52, 53) have suggested that the structure of the SC which is different in heterochromatic and eu chromatic regions may be a factor in the facilitation of crossovers in some regions of the bivalent but not in others. The Drosophila data give general support to the possibility that the site of a crossover may be recognized by an SC-associated modification. SC modifications can also be examined to advantage in mammalian spermatocytes and oocytes. The chromosomes are short, the SCs are well defined, and the chiasmata are visible with the light microscope. So far, rat, mouse, and hamster spermatocyte SC have been shown to have SC nodes which are well defined in either sections (Figure 3) or in water-spread preparations (41, 44, 54, 55). The SC node in this case is a solid bar roughly as long as the SC is wide, and it can be found along the central element, or across the SC (41). Moses (44) reports that in spread hamster sper matocytes there is seldom more than one SC node per SC and that when one is found on a SC the remainder of the SC complement shows them as well. He notes that although they are strongly implicated in crossing-over their role in the process is at present not understood. In reconstructions from serial sections of rat spermatocytes it was re ported that one nucleus had 19 SC nodes and another 22 (41). These were evenly distributed among the SCs (Poisson Chi-square, p
Further
Quick links to online content Ann. Rev. Genet. 1978. 12:433-50 Copyright © 1978 by Annual Reviews Inc. All rights reserved
ULTRASTRUCTURAL STUDIES
+3145
Annu. Rev. Genet. 1978.12:433-450. Downloaded from www.annualreviews.org by Lomonosov Moscow State University on 11/19/13. For personal use only.
OF CHIASMA DISTRIBUTION Peter B. Moens Department of Biology, York University, 4700 Keele Street, Downsview, Ontario, Canada M3J IP3
CONTENTS INTRODUCTION ........................................................................................................ SC NODES AT CROSSOVER SITES ........................................................................ SC REMNANTS AT CROSSOVER SITES ................................................................
SCs AND THE ACHIASMATIC CONDITION . SC COILING MODELS FOR THE REGULATION OF CHIASMA DISTRIBUTION
......... ............................................
................................................................................................................ ..............
433 436
440 441 443 444
INTRODUCTION
The frequency and distribution of crossovers are precisely regulated in the meiotic prophase nucleus. For a given species the number of crossovers per nucleus deviates little from the average (1), but differences may exist be tween male and female reproductive cells of the same species (2-4). Local races may have slightly different averages (1, 5) and individual differences can be caused by mutations (6--8), B-chromosomes (9-12), chromosome rearrangements (13, 14), amounts of heterochromatin (15-20), or environ mental factors (21-24). Within a given nucleus the crossovers are uniformly rather than ran domly distributed among the bivalents (25). Also within a bivalent the crossovers are not randomly distributed. The phenomenon whereby one crossover reduces the probability of another one occurring nearby is termed crossover position interference (25). The frequency and distribution of cross overs can be measured directly by observation of chiasmata at the diplotene stage of meiotic prophase. It can be measured indirectly from recombinant progeny in the offspring of appropriate crosses. 433
0066-4197178/1215-0433$01.00
Annu. Rev. Genet. 1978.12:433-450. Downloaded from www.annualreviews.org by Lomonosov Moscow State University on 11/19/13. For personal use only.
434
MOENS
In an earlier review of the regulation of crossing-over, Lucchesi &Suzuki (13) discuss a variety of models designed to explain unusual crossover distribution patterns. The models take into account factors such as position effects, bouquet arrangement, centromere repulsion, chromosome coiling, and a competition for some substance, active in crossing-over, which is present in limited quantities in the nucleus and can diffuse along the biva lent. The authors favor the view that the different effects are a function of the timing of meiotic events such as synapsis, crossing-over, and segrega tion. For example, a shortened period of synapsis would reduce the possibil ity of a crossover occurring while a longer period would increase the probability of a crossover taking place. Since the writing of that review, the study of meiotic chromosome behav ior has acquired an additional perspective which originated with the discov ery of the synaptonemal complex (SC) by Moses (26). It has become apparent that meiotic crossing-over invariably occurs in the presence of the SC (27, 28) and models of crossing-over now attempt to integrate the SC in the process (28-32). The expectation is that the structure and behavior of the SC and associated structures may provide a clue as to the timing and distribution of crossing-over along the bivalent at meiotic prophase. The interpretation of the variousSC modifications in relation to crossing over can be based not only on their correlations with chiasma frequency and distribution, but also on their possible function in the process of recombina tion itself. Some of the relevant characteristics of models of recombination are therefore briefly repeated here. Where two homologous chromatids are closely associated, they can become involved in an exchange of DNA strands through one of several mechanisms (30, 33-35). The exchange point can travel along the chroma tids for distances of several hundred base-pairs, thus causing one or both of the chromatids to have an extended region of "hybrid DNA" (25, 29, 35-37). Genetic evidence indicates that these exchanges are formed ran domly but in limited numbers along the length of the bivalent (38). Break age of the exchange DNA strands at this point leaves the parental gene sequences intact with possible postmeiotic segregation or conversion for the region of hybrid DNA. To produce a reciprocal crossover it is necessary to rotate one chromatid 1800 relative to the other one. Following a second 1800 rotation in a plane perpendicular to the first, the nonexchange strands are now the ones that cross over. A cut of these strands produces a reciprocal crossover for outside markers with conversion or postmeiotic segregation for the region of hybrid DNA (35). Such reciprocal crossover events interfere with the occurrence of other reciprocal events nearby, but not with a nearby non reciprocal event (29) (Figure 1). The strand isomeration stage may be a necessary intermediate of reciprocal as well as nonreciprocal events (36, 37),
435
Annu. Rev. Genet. 1978.12:433-450. Downloaded from www.annualreviews.org by Lomonosov Moscow State University on 11/19/13. For personal use only.
CHIASMA DISTRIBUTION
making the first 1800 tum compulsory. A different model has two exchange points and does not require strand isomeration to produce a reciprocal event (30, 39). In his discussion of crossover position interference Holliday (29) has pointed out that models of recombination at the molecular level do not predict or explain interference between crossovers. He postulates that a DNA binding protein is required to stabilize a crossover, possibly in the shape of a recombination nodule (40) (Figure 2). The protein is contained within the SC, and at the site of a crossover the protein is depleted in that region and no crossovers can be formed nearby. A more mechanistic model (41, 42) can be designed on the grounds that purely structural chromosome rearrangements can bring about changes in crossover frequencies and distri bution. The stabilization of an exchange event may take place by way of a reorganization of the lateral elements along the crossover strands. Such a reorganization produces coiled SCs or, where the SCs are coiled prior to lateral element realignment, it can result in a reduction ofSC coiling (Figure 3). In this article I review the various SC modifications that may have a bearing on crossing-over and on position interference.
I ..... �
---II
AX8 ��: )0� .A(
__ A ..... �B
. 1--
C --1 ---"'-- 0
C
7
�
�
0
------,--,1:1--:=::_-===: :::-- ---1-: Figure J
Genetic exchange model (35). Only two of four chromatids are diagrammed. The ' DNA strands or the one chromatid are drawn wide and the strands of the nonsister chromatid are drawn narrow. The nicks are indicated by gaps in the lines. At the sites of the initial nicks an exchange of strands is established. The exchange point
can
travel along the two DNA
strands so that regions of hybrid DNA are formed. The crossed strands can be cut so that the chromatids become separated without signs of recombination between flanking markers, but gene conversion may occur in the hybrid region. Alternatively, the strands can be flipped over so that the nonexchange strands become the crossed strands. A cut now results in a classic crossover event with recombination of flanking markers. Conversion can still occur through excision and repair of mismatches in the hybrid region.
--I
Annu. Rev. Genet. 1978.12:433-450. Downloaded from www.annualreviews.org by Lomonosov Moscow State University on 11/19/13. For personal use only.
436
MOENS
Figure 2a,b
A synaptonemal complex node, N, of the type observed in Drosophila oocytes
and named recombination nodules (40). The node, N, is shown in two consecutive sections of a crane fly spermatocyte. Also shown are a synaptonemal complex Sc. the nucleolus NO, and the lateral elements at the site of the node, arrowheads. The scale bar is 1 p.m.
SC NODES AT CROSSOVER SITES
The most extensive observations of the correlation between chiasmata and SC modifications has been reported by Zickler (43) in the fungus Sordaria macrospora. Light microscopy of cells in diplotene show that there are three to four chiasmata in each of the long bivalents ( 13 and 11 J-Lm). The four medium-sized bivalents (7 J-Lm) have two to three chiasmata, and chromo some No. 7, the shortest one ( 6 /Lm), has one chiasma. The average was 18 chiasmata per nucleus for 20 nuclei scored. At the electron microscope (EM) level, in completely reconstructed pachytene nuclei the SCs contain one to four dense bodies in the central element of the SCs. One nucleus had 17 such SC nodes and another had 21. In two diplotene nuclei theSCs have mostly disappeared but several of the remaining short stretches show SC
437
Annu. Rev. Genet. 1978.12:433-450. Downloaded from www.annualreviews.org by Lomonosov Moscow State University on 11/19/13. For personal use only.
CHIASMA DISTRIBUTION
Figure 3a-f Five consecutive sections of a rat spermatocyte showing a synaptonemal com plex, SC; an SC node, N; the centromeric region of a bivalent, CE; and the nuclear envelope,
NE The node is clearly of a different type from the ones found in diptera (Figure 2) and fungi. It appears to be a bar which connects the lateral elements and lies to one side of the SC. The twists or coils of the SC are shown in the lower SC; all are counterclockwise. The scale bar is 1 p.m.
Annu. Rev. Genet. 1978.12:433-450. Downloaded from www.annualreviews.org by Lomonosov Moscow State University on 11/19/13. For personal use only.
438
MOENS
nodes-18 and 19 respectively. In mouse spermatocytes the SC remnants are at the site of a diplotene chiasma (44, 45) and by extrapolation this may imply that the Sordaria SC nodes are at the site of a crossover. A correlation between SC nodes and crossovers is further suggested by the distribution of the SC nodes. Pooling of bivalents No. 3 to No. 6 inclusive for the three pachytene nuclei gives a total of 30 SC nodes for 12 bivalents. The average is 2.5 per bivalent with a variance of only 0.81, indicating that theSC nodes are uniformly rather than randomly distributed as is expected of chiasmata position interference. Because of the small sample size, however,the distri bution does not differ significantly from a Poisson distribution,p 0.20. Zickler (43) shows that the SC nodes appear before synapsis is completed for all chromosomes. This should not be interpreted that the SC nodes are formed during zygotene. In this and many other fungi the meiocyte is formed by the fusion of two haploid nuclei that are already well into meiotic prophase, pachytene judging by the condensed state of the chromosomes, at the time of fusion (46, 47). The type of SC nodes analyzed by Zickler (43) had previously been reported in two other ascomycetous fungi: Neurospora crassa by Gillies (48) in the yeast Saccharomyces cerevisiae by Byers & Goetsch (49). They have also been reported in Chlamydomonas (50), maize (16), and in Ascaris (51). Unlike tlie Sordaria data, these observations are somewhat sporadic and they do not clarify the correlation betweenSC nodes and crossing-over. If, all the same, there is a causal relationship, the SC nodes by themselves do not provide a clue to the mechanism that regulates their numbers and distribution. Recently the SC nodes in Schizophyllum commune and N. crassa have been described in detail (51a, SIb). In N. crassa Gillies reports an average of 19 recombination nodules per pachytene nucleus (based on 8 nuclei) and the observed number of crossovers from genetic studies is about 18 per nucleus. The average distance between nodules is about 3 p.m but some are as far as 11 p.m apart and in one case 3 nodules were present in a 1 Jl.m stretch of SC. The suspected centromeric region has relatively few nodules, while the distal portions of the SCs have a slight excess. As in Sordaria (43) the nodules appear while chromosomes are still pairing and they stay late, till diplotene, when most of the SCs have disappeared. In S. commune less is known about total chiasmata or crossovers per nucleus so that the nodules cannot be related directly to reciprocal recombi nant events. Their numbers and distribution, however, follow a similar pattern as described for Sordaria (43) and Neurospora (51b). Six nuclei with an average of 29 p.m of SC per nucleus had an average of 22 nodules per nucleus. Nine smaller nuclei with 20 p.m of SC per nucleus had 13 nodules per nucleus. The longer bivalents had one nodule per 2.2 p.m ofSC and the shorter bivalents had one nodule per 1.1 p.m of Sc. =
Annu. Rev. Genet. 1978.12:433-450. Downloaded from www.annualreviews.org by Lomonosov Moscow State University on 11/19/13. For personal use only.
CHIASMA DISTRIBUTION
439
A different type of SC node was reported by Carpenter (Figure 2) (40) in Drosophila melanogaster oocyte nuclei. These are large spheres, three to four times as large as the nodes of Sordaria, located on top of the SC somewhat like a large ball on a ladder. Carpenter named this structure a recombination nodule (RN), to denote its correlation with the frequency and distribution of crossing-over in female Drosophila. In a sequence of early to late pachytene stages, she reports oocyte nuclei with 0, 0, 0, 4, 5, 3, and 4 RNs respectively. The total genetic map length of 9 Drosophila is 280 units, which, when divided by 50, gives an estimate of 5.6 reciprocal crossover events per nucleus. There are 5 arms available so that the expected frequency is 1. 12 crossovers per arm. Accordingly, 2 1 of the 23 recon structed SC arms had one RN each, and two arms had two RNs each. In the latter case they were well separated as predicted by positive crossover position interference. The RNs were found in the euchromatic regions only, which is also in agreement with the observed lack of crossing-over in heterochromatic regions. Carpenter & Baker (52, 53) have suggested that the structure of the SC which is different in heterochromatic and eu chromatic regions may be a factor in the facilitation of crossovers in some regions of the bivalent but not in others. The Drosophila data give general support to the possibility that the site of a crossover may be recognized by an SC-associated modification. SC modifications can also be examined to advantage in mammalian spermatocytes and oocytes. The chromosomes are short, the SCs are well defined, and the chiasmata are visible with the light microscope. So far, rat, mouse, and hamster spermatocyte SC have been shown to have SC nodes which are well defined in either sections (Figure 3) or in water-spread preparations (41, 44, 54, 55). The SC node in this case is a solid bar roughly as long as the SC is wide, and it can be found along the central element, or across the SC (41). Moses (44) reports that in spread hamster sper matocytes there is seldom more than one SC node per SC and that when one is found on a SC the remainder of the SC complement shows them as well. He notes that although they are strongly implicated in crossing-over their role in the process is at present not understood. In reconstructions from serial sections of rat spermatocytes it was re ported that one nucleus had 19 SC nodes and another 22 (41). These were evenly distributed among the SCs (Poisson Chi-square, p
