karyotype analysis of the plant- …karyotype analysis of heterodera 229 a large aggregate of...

J. Cell Sci. 43, 225-237 (1980) 225Printed in Great Britain © Company of Biologist* Limited ig8o

KARYOTYPE ANALYSIS OF THE PLANT-

PARASITIC NEMATODE HETERODERA

GLYCINES BY ELECTRON MICROSCOPY.

II. THE TETRAPLOID AND AN ANEUPLOID

HYBRID

PAUL GOLDSTEIN* AND A. C. TRIANTAPHYLLOUDepartments of Plant Pathology and Genetics, respectively,North Carolina State University, Raleigh, North Carolina 27650, U.S.A.

SUMMARY

In the plant-parasite nematode Heterodera glycines, 2 forms, in addition to the diploid(9 bivalents), have been isolated and analysed: a tetraploid (18 bivalents) and an aneuploid(14 bivalents, hybrid between the diploid and the tetraploid). Observations on the formation oftheir karyotypes indicates normal and non-homologous pairing. Eighteen normal synaptonemalcomplexes (SC) are present in pachytene nuclei of the tetraploid. Two of the SCs are enclosedin a large heterochromatin mass that is displaced to one side of the nucleus. Such a mass hasnot been observed in the diploid or the aneuploid. Another 2 normal SCs of the tetraploid haveeach a 'modified SC region' (MSC) within which the SC appears disorganized. The aneuploidhas 14 SCs that are unattached at either end. Only 25 % of the karyotype length is normal in theappearance of the SCs. The rest can be traced by the presence of disorganized SC material andcondensed chromatin. Four MSCs are present in the hybrid nuclei. The possible role of theMSCs in the sex determination system is discussed.

INTRODUCTION

Cytogenetic studies have revealed different patterns of reproduction in the plant-parasitic nematodes (Triantaphyllou, 1971, 1979)- Some forms undergo meiosisduring gametogenesis and reproduce by cross-fertilization (amphimixis) or, in theabsence of males, by meiotic parthenogenesis. Other forms do not undergo meiosisand reproduce exclusively by mitotic parthenogenesis. Analyses of the synaptonemalcomplexes (SC) of pachytene nuclei have revealed certain structural modifications ofthe SC; e.g. decondensed chromatin regions, SCs without distinct central elements,etc., which appear to be peculiar to nematodes. These modifications may representsteps in the evolution of the ameiotic type of maturation of gametocytes and theestablishment of mitotic parthenogenesis (Goldstein & Triantaphyllou, 1978 a, b).

To characterize further the SC complement of plant-parasitic nematodes, adiploid amphimictic species, Heterodera glycines was analysed in the first part of thisstudy and its 9 SCs were reconstructed by electron microscopy of serial sections

* Present address: Department of Biology, University of North Carolina at Charlotte,Charlotte, NC 28223, U.S.A.

226 P. Goldstein and A. C. Triantaphyllou

(Goldstein & Triantaphyllou, 1979). We selected H. glycines because in addition tothe prevalent diploid amphimictic form, a tetraploid (18 bivalents) form was identifiedrecently (Triantaphyllou & Riggs, 1979). Crosses between the diploid and thetetraploid forms have yielded aneuploid hybrids (14 bivalents) which are viable andhave been propagated for many generations (Triantaphyllou, unpublished). The studyof all these forms could provide useful information about the behaviour of the SC indifferent states of ploidy, a situation often encountered in plant-parasitic nematodes.Chromosome pairing in diploid, triploid, and autotetraploid forms of Bombyx morihas also been analysed by electron microscopy and revealed 2 distinct phases ofchromosome pairing (Rasmussen, 1976, 1977; Rasmussen & Holm, 1979).

In the present study, we have illustrated the pachytene karyotype of the tetraploidand aneuploid forms of H. glycines following serial sectioning and 3-D reconstruction.We have also attempted to clarify the mechanism of formation of tetraploid andhybrid karyotypes.

MATERIALS AND METHODSThe tetraploid population of Heterodera glycines used in this study is the one isolated by

Triantaphyllou & Riggs (1979). It has been propagated on soybean seedlings in the greenhousesince its discovery in 1973.

The aneuploid hybrid was produced by crossing females of the diploid population used inthe first part of this study (Goldstein & Triantaphyllou, 1979) with males of the tetraploidpopulation. A cytological study by light microscopy established that the tetraploid had iSbivalents, whereas, the hybrid isolate of the present study had 14 bivalent chromosomes atmetaphase of the first maturation division of the oocytes. Both tetraploid and aneuploid formswere viable and reproduced exclusively by cross-fertilization.

The procedures and methods employed for electron microscopy were those reported in thestudy of the diploid form (Goldstein & Triantaphyllou, 1979). In addition to the nuclei com-pletely reconstructed, numerous nuclei were also examined under the electron microscope tocheck for consistency in number of modified synaptonemal complexes present. Thus, the datain Tables 1-3 are considered to be typical and representative of the tetraploid and aneuploidhybrid forms.

RESULTS

The general morphology of the gonad and the formation and behaviour of thesynaptonemal complexes in pachytene nuclei in the tetraploid and hybrid forms aresimilar to those of the diploid form, as described in the previous article (Goldstein &Triantaphyllou, 1979).

Tetraploid

Three pachytene nuclei were completely reconstructed from serial sections:nuclei no. 1 and no. 1 a were adjacent nuclei in the same ovary; nucleus no. 2 was froma different female (Table 1). There are 18 SCs in each pachytene nucleus (compared to9 in the diploid). Pairing of the chromosomes appears to be complete. The chromatinalong the SC is distinctly more condensed than in the diploid (Fig. 2). One end ofeach SC is attached to the inner nuclear membrane and the other end is free in the

Karyotype analysis of Heterodera

Table i. Pachytene chromosome lengths of female tetraploid (18 bivalents)from reconstruction of synaptonemal complexes

227

SCno.

1

2

3456789

1 0

1 1

1 2

13141516

1718

Totalkary-otype/ i m

Nu-clearvol.,/tm'

No.

NucleusA

Length,fim

5°5-76-37-8*7 98-o*8 08 1

1 0 31 0 31 0 5

u s13-2'1 6 12 4 0

2 6 22 8 1

2467

130

2

M S C

no. 1

Rel.,0//o

2-O

2-3

2-53 '23 '2

3-23-2

3-34 ' 24 2

4'34 75 46-59'7

1 0 611-4

1 6 1—

—

—

NucleusA

Length,/ i m

3 65-66-38 - i *

8-4*8-89-8

10-9n - 6i4-5i6-Sa

1 6 61 6 92 I - I

2 2 72 5 12 6 0

39-901"272-4

139

2

no. 1 a

Rel.,0//o

i - 32 ' I

2-33-0

3 13 2

3 64-0

4 '35-36 16 16 2

Tl8-39 2

9-514-6—

—

—

NucleusA

t

Length,/ i m

6 2

7-4*7 99-0*

1 0 4

I I - 8

1 2 9

i4-31 5 4i5-81 6 51 7 1

18-719-9°2 2 8

23-727-528-90'1'

2862

1 3 3

2

no. 2

Rel.,0//o

2-2

2 62 8

3 13 64 14'5S°5'45'55-85 96-56 98 0

8-39 7

IO'I

—

—

—

Average>

Length,/ i m

4'96 26-88-38-99 5

IO-2I I I

1 2 4

i3'514-615-21 6 21 9 023-225-027-23 6 2

268-4

134

2

Rel.,/o

i -8

2-3

2 53 13 33'53 74 i4'75 15'45'56 17-0

8-79 4

IO-21 3 6—

—

—

Rel. = relative length.0 Modified synaptonemal complex (MSC).• SCs associated via chromatic mass.

6 Nucleolar organizer region (NOR).

nucleoplasm (Fig. 3). Attachment of the SCs appears to be random and no bouquetformation is observed. The relative length of the SCs of the 3 nuclei studied variesconsiderably more than in the diploid (Table 1) (Goldstein & Triantaphyllou, 1979)since in the diploid the individual SC lengths were similar. The total karyotype lengthis on the average 268 /im, i.e. about the same as in the diploid.

There are 2 modified SC regions (MSC) in each tetraploid nucleus. These areidentical in structure to those observed in the diploid, as they consist of a hetero-chromatic ball within which the SC appears disorganized. However, the position ofMSC no. 2 in relation to the attachment of the SC on the nuclear envelope is differentfrom that of the diploid (12-22% as compared to 53-55%) (Table 3, p. 232). In eachnucleus, one of the MSCs is located on the longest SC, whereas the other is located ona mid-sized SC (Table 1). Both MSCs appear to be of similar sizes.


t

Karyotype analysis of Heterodera 229

A large aggregate of chromatin is always displaced to one side of the pachytenenucleus (Fig. 9.A-F, p. 235). Within this chromatic mass, there are two separate SCswhich vary in length from 7-4 to 9-0 fim (Fig. 9F, Table 1).

The nuclear morphology appears slightly irregular as the nuclear envelope isconvoluted and the mitochondria are not normally shaped because of irregularmembrane structure (Fig. 2). The nuclear volume is on the average 134 fim3 (Table 1)and this is about one-half the volume of the nucleus of the diploid form.

Hybrid

There are 14 normal SCs, i.e. are attached to the nuclear envelope, in each of the2 pachytene nuclei of different females studied and the lengths of the SCs range from37 to 94-4 fim (Table 2, Fig. 6). There is no apparent bouquet arrangement of SCends (Fig. 4). The relative lengths of the SCs in the 2 nuclei are not the same (Table 2).The average total karyotype length is 434 fim. One end of each SC is attached to theinner nuclear membrane and the other end is free in the nucleoplasm (Fig. 4). Thelocation of the nucleolar organizer region (NOR) on the SC suggests that either endof the SC is able to attach to the nuclear envelope (thus, in nucleus no. 1 the NOR islocated 84% from the attached end of the SC while in nucleus no. 2 the NOR islocated 16% from the attached end of the SC, or 84% from the free end; Table 3).There are segment(s) of SCs that are not attached to the nuclear envelope and are freein the nucleoplasm (Fig. 4). In one nucleus, there is 1 piece, 24-9 fim long and in thesecond nucleus there are 2 pieces, IO-I and 28-4 fim in length (Table 2). Such un-attached SCs were not observed in the diploid or tetraploid.

SC formation is not normal throughout the entire length of the chromosomes(Fig. 1). Only 108 fim, i.e. 25%, of the total karyotype length of 434 fim consists ofSCs of normal structure (Table 2). Along the rest of the bivalents, the SC material ispresent, but poorly organized (Fig. 1). We recognize that oblique views of normal SCsmay occasionally appear 'disorganized', but this is not the case here.

Seven distinct heterochromatic masses (HM) were observed in each nucleus witha total volume of approximately 1-9 fim3. In nucleus no. 1, 4 HMs are on differentSCs and 3 are located on the longest SC (94-4 fim), on which 2 MSCs and the NOR(Nucleolar Organizer Region) are also located. In nucleus no. 2, 6 HMs are located on6 different SCs and one on the unattached segment. The HMs are always inter-stitially located on the SCs and never at the ends.

Four modified SC regions are present in each hybrid pachytene nucleus (Table 2),similar in structure to those observed in the diploid and tetraploid. The relative

Fig. 1. Oocyte pachytene nucleus from the Heterodera glycines hybrid. There arefew regions where normal synaptonemal complexes {sc) are present. Most of thechromatin along the sc is highly condensed (ch) and the sc material appears dis-organized (arrows). Bar, o'3 fim.Fig. 2. Oocyte pachytene nucleus from tetraploid. The chromatin along the sc is morehighly condensed than in the diploid. The nuclear (n) and mitochondrial (m) morpho-logies are irregular, ne, nuclear envelope; nu, nucleolus. Bar, 04 fim.

230

2*

12'

P. Goldstein and A. C. Triantaphyllou

15

S13*. \

V A ' 32^9

fcS-,14

,10

MSC

MSC

14'

AfSC

12

T

\ 13

MSC

1 ?-7. . .„•5 1 1

2 r^._n.7•3 11-03 11-94 irm.,3. ,

c a 12-7c . a. 130b r^TT-..,5.2

7 137/ 23-08 25-4q 1 9 9

» 30-510 32.9

11 35.2

51-012 61-8. „ 52-6 «13 - - - b n-8M, . 94-4 » ' a» b14 - -81-3

6

Karyotype analysis of Heterodera 2 3 1

Table 2. Pachytene chromosome lengths of female aneuploid hybrid (14 bivalents)

from reconstruction of synaptonemal complexes

1

SC Length,no. fim

i 3'72 7-7"X I I *0

4 11'75 i2-7°6 1306

7 13-78 18-59 199

10 2 7 811 38-51 2 5 1 0

13 52614 944ai 'Unat- 24-9tachedseg-ments)

Total 401-1karyo-type,fim.

Nu- 460clearvol.,

Total 103-6SCforma-tion,fim

No. 4MSC

Rel. = Relative

NucleusA

Rel.,0//o

0 9

1-92 72 93-23 2

3-44-65'0

6 99 6

12-71 3 1

'•" 25-56 2

—

—

length.

no. 1

sc %formation

7-83683661 2 6

3 7 61 5 2

3 2 61 7 1

3 3 22 9 1

3 0 0

I7'229-025-02 9 0

—

—

25-8

a Modified synaptonemal complex

Nucleus nc

Length,fim

9 6H-7-1 1 9

1 3 1

1 4 41 5 22 3 0

25-43 0 53 2 935-26i-8"62-4"ia'i

81-328-4IO-I

4669

45o

112-4

4

(MSC).

Rel.,% )

2 - 1

2-52 62-8

3-13 2

4 95-46-57-07-6

1 3 2

" 13-31 8 0

6- i2 2

1. 2

sc% Length,formation fim

4 2 61 0 9

21-982-9i3'8i9-71 7 0

43 03i-52 I - I

14-82O-O

25'81 9 72O-8

1 4 1

—

—

24-1

h Nucleolar c

6659-70

" •4512-4013-5514-10i8'3521-9525203O-35368556-4057-5087-852 I - I O

434

455

108

4

organizer

AverageA

Rel.,% :

1 5 92-222 6 52-87

3 1 33 2 54-185 035'756 9 98-57

12-9813-2421-48

4 8 0

—

—

sc%formation

2 5 2

2 3 929-347-82 5 8I7-52 4 83 0 1

32-42 5 122-41 8 627-422-42 1 3

—

—

25-0

region (NOR).

Figs. 3, 4. Reconstruction from serial sections of sc ends from the tetraploid andhybrid, respectively. There is no bouquet arrangement. ( A ) represents the end ofa chromosome that is attached to the nuclear membrane, while (/•*) is the free end inthe nucleoplasm. MSC represents location of modified sc regions, x , represents theends of the unattached segment. Bar, 0 4 /im.

Figs. 5,6. Reconstructed karyotypes of tetraploid and hybrid. The lengths are in fim.a = modified sc region, b = nucleolar organizer region. Nucleus no. 1 is representedby straight line; no. 16 (in tetraploid) by dotted line and no. 2 by a dashed line.


Table 3. Relative position of modified synaptonemal complex (MSC) andnucleolar organizer region (NOR) on synaptonemal complexes

TetraploidMSC no. 1% rel. val.MSC no. 2% rel. val.NOR% rel. val.

Aneuploid hybridMSC no. 1% rel. val.MSC no. 2% rel. val.MSC no. 3% rel. val.MSC no. 4% rel. val.NOR% rel. val.

Distance

No. 1

0 8 0

68-3

2 1

1 9 4

49

3 93°23-52556-26 0

3-2

2579784

to nuclear envelope, fim.

No. 2

2-60133 4

1 2

1 2 9

45

2-52 1

5 1 182

5799 2

1 9 3

319 8

16

No. \aTetraploid

5-6143 6

2 2

1 6 842

position of the MSCs on the SCs are variable (Table 3). In each nucleus, there is 1 SCon which 2 MSCs and the NOR are located. The other 2 MSCs are on different SCs(Table 2).

DISCUSSION

Polyploidy is rare among cross-fertilizing animals. The tetraploid studied hereprobably represents an autotetraploid of recent origin (Triantaphyllou & Riggs, 1979).Irregularities in chromosome pairing are observed at metaphase I (20% of nucleiobserved) when quadrivalents, trivalents and univalent chromosomes are seen inaddition to bivalents. Still, telophase I is usually normal and metaphase II andanaphase II are always normal (Triantaphyllou & Riggs, 1979). In spite of the presenceof multivalents at metaphase I, 18 normal SCs were observed in the 3 pachytenenuclei studied suggesting that formation of SCs and therefore, pairing of homologouschromosomes, is regular. No multivalent associations were observed at pachytene.Such associations may occur secondarily, at a later stage (e.g. diakinesis), due toattraction of homologous or homeologous chromosomes, and do not represent truepairing. In autotetraploid Bombyx females, multivalent associations are absent atmid-late pachytene and only bivalents are formed (Rasmussen & Holm, 1979).

The tetraploid has twice the number of SCs as the diploid but the total karyotypelength is similar in the tetraploid and the diploid. This suggests that a possible way of


derivation of the form with 18 bivalent chromosomes (18 SCs) may be throughfragmentation of the chromosomes of the diploid (9 bivalents). However, the tetra-ploid form has more than 1-5 times the amount of DNA per nucleus than the diploid,and has juveniles and adults of significantly larger body measurements, as would beexpected from a polyploid (Triantaphyllou & Riggs, 1979). It seems that comparisonsof SC lengths of the tetraploid and diploid may be misleading since they may havebeen measured at different stages of pachytene. Drastic shortening of SC lengths atlate pachytene have indeed been reported in Drosophila (Carpenter, 1975); Zea(Gillies, 1973; Maguire, 1978a) and Chinese hamster (Moses, Slatton, Gambling &Starmer, 1977). Furthermore, chromatin is much more condensed in the tetraploidthan in the diploid. It is very likely that chromatin distribution is related to theorganization of the SC, and consequently, to the shortening of the total SC length inthe tetraploid.

In the diploid, the NOR and the MSCs were located on different SCs (Goldstein &Triantaphyllou, 1979). In the tetraploid, the NOR and 1 MSC are located on the sameSC. This observation strongly suggests that a chromosomal translocation has occurredin the tetraploid in addition to the doubling of the chromosome number.

If the tetraploid has arisen from the diploid via doubling of the chromosomes,4 MSCs (twice the number present in the diploid) would be expected to be found inthe tetraploid, rather than the two observed. However, a large chromatic association,that is displaced to one side of the nucleus and has 2 SCs within it, has been observedin the tetraploid nucleus, but not in the diploid or hybrid nuclei. It is possible that the2 SCs associated with this chromatic mass are actually the 2 missing MSCs, expressedin a different form. This structural change may indicate a change in function, possiblyinactivation of the chromatin associated with them. Furthermore, it is possible thatthe MSCs, in general, are the segments of the karyotype where the sex-determiningchromatin is located. The MSC is a unique SC structure and alteration of SC structureusually implies a function. Thus, the chromatic mass, with its 2 SCs would then bethe extra sex chromatin in an inactive state. In many other organisms, extra sexchromosomes are not advantageous and may be selectively inactivated or eliminated(White, 1973). The preservation of the normal (diploid) complement of sex chromatinwould aid in the viability of the tetraploid and would allow normal sex expression(males and females, but no intersexes) and amphimictic reproduction. This isespecially true in the aneuploid hybrid, which otherwise would have been unbalancedconcerning sex factors.

Formation of the hybrid karyotype

The cross between the diploid and tetraploid yielded a viable aneuploid hybridwith 14 SCs in the pachytene nucleus. Approximately 25% of the total karyotype ofthe hybrid consists of regions where the SC has normal structure. Along the rest ofthe chromosomes (75 %), the SCs are poorly organized (Figs. 7, 8). To our knowledge,this situation has not been encountered in any other organism. However, it is knownthat the process of pairing of homologous chromosomes and the formation of SCsduring the first meiotic prophase is influenced by many factors including DNA-to-

16 CEL 43


8

Fig. 7. Reconstruction of sc no. 11 in hybrid nucleus no. 1 detailing the distributionof normal (:Ht) and abnormal (-^) sc regions. Approximately 25 % of the bivalentis normal in sc structure. Bar, 0-4 fim.Fig. 8. Possible mechanism for formation of the unattached segment in the hybrid.Two chromosomal pairs (A, B) of uneven length have extended unpaired axial cores(ax): These unpaired axial cores may pair for a short distance. Since unpaired axialcores are not visible in this nematode, the regions where they do pair form theapparent unattached segment (5).

DNA binding, nuclear envelope-mediated chromosome movement and the uniquetiming of synaptic competence (Moens, 1973). The condensation of chromatin duringprophase I is specific in its rate and extent of coiling and varies from species to species.It may be that the entire segmental association of chromatin with the SC wouldconceivably be in a state of flux (Maguire, 1978a) with some localized areas (areaswhere chiasmata occur) that might remain constant. Due to the extensive condensa-tion of the chromatin in the tetraploid at prophase I, compared to the diploid, thepairing of homologous chromosomes and formation of SCs may not be synchronousin the 2 forms. This possible lack of synchrony of condensation of chromosomes mayresult in hybrid bivalents with poorly formed SCs since binding sites would only, bychance, be in register. However, short stretches of SC at cross-over sites could serve

Fig. 9. The chromatic association that is displaced to one side of the tetraploidpachytene nucleus is absent from the diploid and hybrid pachytene nuclei. Fig. 9 A-Eare 5 serial sections through the chromatic association and F is the reconstruction.Two separate scs are located within the mass, np, nuclear pore, x 32500 approx.


* & %

D


the demands for homologous pairing (Holliday, 1977) and recombination (Maguire,19786).

An interesting feature of the karyotype of the hybrid is that 1 of its SCs has 2MSCs and the NOR located on it. This SC apparently has resulted from the associa-tion of 2 SCs, one from one parent (with one MSC and the NOR) and another fromthe other parent (with a MSC). Therefore, some chromosomal rearrangements appearto have occurred during the establishment of the aneuploid hybrid. This then wouldgenerate non-homologous pairing of chromosomes. The variable location of the MSCsin the hybrid (Table 3) suggests non-homologous pairing.

Formation of unattached segments of SCs

Unattached pieces of SCs are not present in the diploid or tetraploid. However,such pieces were observed in nucleus no. 1 of the hybrid (Fig. 9), 1 segment of 24-9 fim;and in nucleus no. 2, 2 segments of IO-I and 28-4/im. These segments are of con-siderable length and would appear as individual chromosomes when viewed with thelight microscope, thus creating haploid chromosome counts of n = 14, 15 and 16.The extent of normal SC formation is 25%, similar to the rest of the karyotype.None of the 4 MSCs are located on the unattached segment. An explanation of thepossible mechanism of formation of these segments is presented in Fig. 8. Pairingbetween 2 chromosomes of different lengths can be assumed. One of the axes wouldextend past the other and pair with another unpaired axis from a differentchromosome, to which it may be homologous. The exact association between suchchromosomes is not defined clearly, since in this nematode unpaired axial cores arenot visible. Therefore, the paired segments would appear ' unattached' to the rest ofthe chromosomes.

We thank Mr Eugene McCabe for valuable technical assistance. Part of this work was donein the laboratory of Dr M. J. Moses, Department of Anatomy, Duke University, and we thankhim for his cooperation, discussions and review of this manuscript. Financial assistance wasprovided by the National Science Foundation Grant DEB 76-20968 A02 to A. C. Triantaphyllouand by the International Meliodogyne Project, Contract no. AID/ta8-C-i234.

Paper no. 6171 of the Journal Series of the North Carolina Agricultural Research Service,Raleigh, N.C.

REFERENCES

CARPENTER, A. T. C. (1975). Electron microscopy of meiosis in Drosophila melanogasterfemales. I. Structure, arrangement, and temporal change of the synaptonemal complex inwild type. Chromosoma 51, 157-182.

GILLIES, C. B. (1973). Ultrastructural analysis of maize pachytene karyotypes by three-dimensional reconstruction of the synaptonemal complex. Chromosoma 43, 145-176.

GOLDSTEIN, P. & TRIANTAPHYLLOU, A. C. (1978a). Occurrence of synaptonemal complexesand recombination nodules in a meiotic race of Meloidogyne hapla and their absence in amitotic race. Chromosoma 68, 91-100.

GOLDSTEIN, P. & TRIANTAPHYLLOU, A. C. (1978 ft). Karyotype analysis of Meloidogyne hapla by3-D reconstruction of synaptonemal complexes from electron microscopy of serial sections.Chromosoma JO, 131-139.

GOLDSTEIN, P. & TRIANTAPHYLLOU, A. C. (1979). Karyotype analysis of the plant-parasiticnematode Heterodera glycines by electron microscopy. The diploid. J. Cell Sci. 40, 171-179.


HOLLIDAY, R. (1977). Recombination and meiosis. Phil. Tram. R. Soc. B 277, 359-370.MAGUIRE, M. (1978a). Change in length of synapsed bivalents during pachytene. Chromosoma

69, 155-162.MAGUIRE, M. (19786). A possible role for the synaptonemal complex in chiasma maintenance.

Expl Cell Res. 112, 297-308.MOENS, P. B. (1973). Mechanisms of chromosome synapsis at meiotic prophase. Int. Rev.

Cytol. 35, 117-132.MOSES, M. J., SLATTON, G. H., GAMBLING, T. M. & STARMER, C. F. (1977). Synaptonemal

complex karyotyping in spermatocytes of the Chinese hamster (Cricetuhis griseus). III.Quantitative evaluation. Chromosoma 60, 345-375.

RASMUSSEN, S. W. (1976). The meiotic prophase in Bombyx mori females analyzed by three-dimensional reconstructions of synaptonemal complexes. Chromosoma 54, 245-293.

RASMUSSEN, S. W. (1977). Chromosome pairing in triploid females of Bombyx mori analyzedby three dimensional reconstructions of synaptonemal complexes. Carhberg Res. Commun.42, 163-197-

RASMUSSEN, S. W. & HOLM, P. B. (1979). Chromosome pairing in autotetraploid Bombyxfemales. Mechanism for exclusive bivalent formation. Carhberg Res. Commun. 44, 101-125.

TRIANTAPHYLLOU, A. C. (1971). Genetics and Cytology in Plant Parasitic Nematodes, pp. 1-32.New York: Academic Press.

TRIANTAPHYLLOU, A. C. (1979). Cytogenetics of root-knot nematodes. In: Root-knot Nema-todes (Meloidogyne species) (ed. F. Lamberti & C. E. Taylor), pp. 85-109. London: AcademicPress.

TRIANTAPHYLLOU, A. C. & RIGGS, R. D. (1979). Polyploidy in an amphimictic population ofHeterodera glycines. J. Nemat. (in press).

WHITE, M. J. D. (1973). Animal Cytology and Evolution. Cambridge: Cambridge UniversityPress.

(Received 30 July 1979 — Revised 13 December 1979)

karyotype analysis of the plant- …karyotype analysis of heterodera 229 a large aggregate of...

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