Interstitial Chiasma Research Articles

Meiotic chromosome behaviour of a newly recorded ant-like spider, Myrmarachne melanocephala MacLeay, 1839 from Manipur, India

Cytological and chromosomal studies of ant-like spiders Myrmarachne melanocephala MacLeay, 1839, were undertaken with 12 males captured alive in the months of January to July 2023 from three habitats. The haploid count of male specimens was observed to be 13: 4 acrocentric (including X chromosome), 6 subtelocentric, 2 submetacentric, and 1 metacentric and showed X0 sex determination mechanisms, so the diploid count of the species was 25 (13+12). The single sex chromosome occupied roughly 4-6 per cent of the nuclear volume prominently in the Interphase – Prophase I stage. The structure of X chromosome in interphase stage was circular-rectangular block. The peculiar shape of rod-shaped X chromosome was maintained from early pre-leptotene stage till the end of the division particularly the Prophase I. The synapsis started early from late leptotene and duration of zygotene was long enough to visualize the perfect ones in late zygotene. There were nine diplotene bivalents with interstitial chiasma.

Meiosis and male fertility in F1 interspecific hybrids (Passiflora vitifolia vs. Passiflora hatschbachii)

Interspecific hybrids can be studied using methodologies in which the male gamete with high reproduction potential, viability, and fertility is prioritized. Passiflora species, with lush, showy, and exotic colors, have great potential for the ornamental plant market. In addition, artificial Passiflora hybrids were developed without many difficulties because of weak reproductive barriers. Thus, meiotic and post-meiotic behaviors were analyzed with 2% acetic carmine staining. Confirmation of interspecific hybridization was performed using SSR markers and GISH technique was used to detect genomic differentiation. The pollen viability of the parental and hybrids genotypes was tested using Alexander`s solution, fluorescein diacetate and in vitro germination tests were performed using culture medium. The meiotic behavior was regular and displayed haploid number n = 9 with nine bivalent (II) chromosomal, and pairing in 90% of the cells in diakinesis. There was a significant difference (p < 0.05) in terminal and interstitial chiasma frequencies. Meiotic irregularities observed were as follows: early and/or delayed chromosomes, disorientation of spindle fibers, transverse spindles, tripolar spindles, and asynchrony; and consequently irregular post-meiotic products were observed: monads, dyads, triads, and polyads. GISH was used in the interspecific hybrids and pairing between homeologous chromosomes, and bivalent and tetravalent formation were observed. From this study, we could conclude that hybrid genotypes are fertile and pollen grains are viable and can be used in breeding programs.

Variability of Chiasma Frequencies in Different Tomato Species

This article presents the results of comparative studies of the frequency and distribution of chiasmata in pollen mother cells (PMCs) in five diploid tomato species, Solanum lycopersicum, S. pimpinellifolium, S. peruvianum, S. habrochaites, and S. neorickii, and one autotetraploid species, S. pimpinellifolium. It was established that under the same growing conditions, the total chiasma frequency in the cell depended on the species. At the same time, the green-fruited species S. peruvianum, S. neorickii, and S. habrochaites differed in distal chiasma frequency, while the red-fruited species S. lycopersicum and S. pimpinellifolium differed in interstitial chiasma frequency. It was shown that the total chiasma frequency in PMCs of plants of one species is a stable index of recombination potential that does not depend on the growing conditions. The redistribution between distal and interstitial chiasmata was found to be more variable, depending on the species, year, geographic growth conditions. In autotetraploid, the chiasma frequency per bivalent was lower than that in diploid S. pimpinellifolium plants, primarily due to interstitial chiasmata, the frequency of which remained at the level characteristic for diploid plants. It was concluded that the recombination plasticity of the tomato genomes was due to the redistribution of chiasmata along bivalents, and not to the change in their number in the cell.

Micro-Evolution in Grasshoppers Mediated by Polymorphic Robertsonian Translocations

This review focuses on grasshoppers that are polymorphic for Robertsonian translocations because in these organisms the clarity of meiotic figures allows the study of both chiasma distribution and the orientation of trivalents and multivalents in metaphase I. Only five species of such grasshoppers were found in the literature, and all of them were from the New World: Oedaleonotus enigma (Scudder) (Orthoptera: Acrididae), Leptysma argentina Bruner, Dichroplus pratensis Bruner, Sinipta dalmani Stål, and Cornops aquaticum Bruner. A general feature of these species (except O. enigma) is that fusion carriers suffer a marked reduction of proximal and interstitial (with respect to the centromere) chiasma frequency; this fact, along with the reduction in the number of linkage groups with the consequent loss of independent segregation, produces a marked decrease of recombination in fusion carriers. This reduction in recombination has led to the conclusion that Robertsonian polymorphic grasshopper species share some properties with inversion polymorphic species of Drosophila, such as the central-marginal pattern (marginal populations are monomorphic, central populations are highly polymorphic). This pattern might be present in D. pratensis, which is certainly the most complex Robertsonian polymorphism system in the present study. However, L. argentina and C. aquaticum do not display this pattern. This issue is open to further research. Since C. aquaticum is soon to be released in South Africa as a biological control, the latitudinal pattern found in South America may repeat there. This experiment's outcome is open and deserves to be followed.

Meiotic behavior in wild and domesticated species of Passiflora

The meiotic behavior of fourteen Passiflora taxa was analyzed. The species were grouped according to the n value (6, 9 and 12) for statistical studies. Some species presented tetravalent associations or univalent chromosomes in diakinesis, bivalent formation prevailing. The qui-square test revealed significant differences in the chiasma frequency among species for n = 9 and n = 6 groups. There was predominance of interstitial chiasmata in almost all studied species. The n = 12 group was the only one whose meiotic behavior was considered similar due to the quantity of chiasmata per cell, tendency of interstitial chiasma localization. Some species presented meiotic irregularities, such as laggard and precocious chromosomes in meiosis I. In telophase II the percentages of meiotic irregularities was low. Irregularities in the spindle orientation were presented in higher percentages in the end of meiosis II, and were also responsible for post-meiotic abnormal products. The irregularities observed during meiosis can have influence on the percentage of sterile pollen grains and success of interspecific crossings in Passiflora species.

Mesabolivar brasiliensis (Moenkhaus 1898) and Mesabolivar cyaneotaeniatus (Keyserling 1891) (Araneomorphae, Pholcidae): close relationship reinforced by cytogenetic analyses

Pholcidae is the most diverse family among haplogyne spiders but only 15 species have been analyzed cytogenetically. These studies revealed that the diploid number varies from 2n 5 15 to 2n 5 32, that there are three types of sex chromosome systems in males (X, X1X2 and X1X2Y), and that the chromosomes are predominantly biarmed. Within the genus Mesabolivar, only Mesabolivar luteus (Keyserling 1891) has been karyotyped, and it showed 2n 5 15 5 14 + X, with all chromosomes being metacentric. In the present work, we characterize the mitotic and meiotic chromosomes of Mesabolivar brasiliensis (Moenkhaus 1898) and Mesabolivar cyaneotaeniatus (Keyserling 1891). Male mitotic metaphases of the two species showed the diploid number 2n 5 17 5 16 + X; oogonial cells of M. brasiliensis showed 2n 5 18 5 16 + XX. In both species, the chromosomes were exclusively biarmed, and the X chromosome was the largest element of the karyotype. Diplotene spermatocytes of the two species exhibited 8II + X and the occurrence of only one terminal or interstitial chiasma per bivalent. In M. cyaneotaeniaus, metaphases II with n 5 9 5 8 + X and n 5 8 were found, indicating the regular segregation of all chromosomes during meiosis I. Mitotic metaphases of M. brasiliensis stained with CMA3/DA/ DAPI revealed GC-rich chromatin in the terminal region of almost all autosomes, especially in pair 2. An earlier revision of the New World pholcids grouped M. brasiliensis and M. cyaneotaeniatus in a ''southern group'' and placed M. luteus in a ''miscellaneous group.'' A molecular study showed a closer relationship between M. brasiliensis and M. cyaneotaeniatus than between M. luteus and either of these two species. The 2n 5 17 found in M. brasiliensis and M. cyaneotaeniatus corroborates this hypothesis, given that M. luteus has a diploid number of 2n 5 15. According to Platnick (2008), the family Pholcidae Koch 1851 includes 999 extant species, and thus constitutes the most diverse family among haplogyne spiders. A phylogenetic analysis based on morphological characters separated pholcids into four clades: ''ninetines,'' ''pholcines,'' ''holocnemines,'' and the ''New World clade'' (Huber 2000). The genus Mesabolivar Gonzalez-Sponga 1998, which ranges from northern South America to northern Argentina, is included in the ''New World clade'' and possesses 45 species (Huber 2000, 2008). Moreover, the genus is divided into four operational groups based on morphological characters: a ''northern group with spines on male metatarsi,'' a ''northern group without spines on male metatarsi,'' a ''southern group,'' and a ''miscellaneous group.'' The species Mesabolivar brasiliensis (Moenkhaus 1898) and Mesabolivar cyaneotaenia- tus (Keyserling 1891), both part of the ''southern group,'' are distributed in southern and eastern Brazil and northern

Tetrad Analysis with Translocation Heterozygotes in Cranberry (Vaccinium Macrocarpon Ait.): Interstitial Chiasma and Directed Segregation of Centromeres

Tetrad analysis of translocation heterozygotes was used in cranberry (Vaccinium macrocarpon Ait.) to study the co-orientation of centromeres during meiosis 1 and its relationship with the frequency of interstitial chiasma. Pollen tetrads from one reciprocal and three cyclical translocation heterozygotes were analyzed and their frequency distribution, as manifest in pollen stainability, was determined. Pollen tetrads were characterized as having four, two, one or zero stained pollen grains. The occurrence of each type was the result of the type of centromere segregation and the presence of crossing over in the interstitial region(s). Based on this information, the predominance of alternate segregation was found to be correlated with a reduction of the crossing over in the interstitial region. Significant differences were found in the tetrad frequency distribution for pollen stainability between years, and for cultivars carrying the same translocation. This variation was correlated with significant differences in the frequency of crossing over between years and among cultivars, which indicates that the estimation of recombination frequencies was influenced by both environment and genotype. The occurrence of translations in cultivars selected from wild populations indicates the possible advantage of maintaining heterozygosity or a block of genes as a linkat through this chromosome aberration in a self-pollinated crop like cranberry.

Meiotic Pairing and Chiasma Frequency in an Induced Trisomic of Grass Pea (Lathyrus sativus L.)

Meiotic pairing association and chiasma frequency were studied in a gamma-ray induced primary trisomic of grass pea (Lathyrus sativus L.). The trisomic was identified by phenotypic abnormalities, early maturity and presence of an extra chromosome (2n+1=15), while occurrence of 7 bivalents was the usual feature in normal diploids (2n=14). In the trisomic, formation of a univalent was more common than that of a trivalent, although bivalent association was predominant. Ring bivalents appeared more frequently than chain or rod configurations. Terminalization of 2 chiasmata in the 2 arms may give rise to ring bivalent, while a single chiasma may terminalize to form a chain. In the trisomic, frequent occurrence of linear or frying pan configuration and occasionally Y-shaped trivalent formation indicated unmodified nature of the extra chromosome; this characterized it as a primary trisomic. Terminalization of chiasmata in both the arms of a chromosome involving its 2 homologous partners may result in linear configuration, while a chiasma in the distal end with another chiasma in the same arm may give rise to frying pan configuration, but an interstitial chiasma accompanied with another chiasma formed at the terminal position distal to former might form Y-shaped trivalent. Configuration of trivalent may thus be influenced by the number and position of chiasmata. Compared to normal diploids chiasma frequency reduced in the trisomic. More frequent occurrence of unequal anaphasic separation of chromosomes may be attributed to reduced pollen fertility in the trisomic.

Estimation of the Highest Chromosome Number of Eukaryotes Based on the Minimum Interaction Theory

According to the minimum interaction theory, the chromosome evolution of eukaryotes proceeds as a whole toward increasing the chromosome number. This raises the following two questions: what was the starting chromosome number of eukaryotes and does the chromosome number increase infinitely? We attempted to provide a theoretical framework to resolve these questions. We propose that the species with n=2 observed in Protozoa, Platyhelminthes, Annelid, Algae, Fungi and higher plants would be chromosomal relicts conserving the karyotypes of ancestral eukaryotes. We also propose that the ideal highest number of eukaryotes ( n max ) can be given by an inverse of the minimum terminal interference distance ( It min ) in crossing-over ( n max =100/ It min ). As It min =0.6 in mammals, n max ≈166. On the other hand, the value estimated by computer simulations is somewhat lower with n max =133–138. Our arguments can be applied to other eukaryotes, if they have a localized centromere and the ratio of total synaptonemal complex/nuclear volume is comparable to that of mammals. We revealed that the index of gene shuffling per karyotypes ( G) by means of the total number of gamete types with different gene combinations can be formulated as G =2 n+ Fxi , where Fxi means interstitial chiasma frequency per cell corresponding to crossing-over mediated by the recombination nodule. The Fxi value increases in proportion to the n value in areas where n<40, but decreases gradually when n>40 and becomes zero when n>83. Therefore, in the ultimate karyotype with n max =166, FXi=0 and G =2 n =2 166, where gene shuffling is guaranteed by the random orientation of chromosomes at the equatorial plate of meiotic metaphase I.

Karyotype, DNA Content and Meiotic Behaviour in Five South American Species ofVicia(Fabaceae)

This paper presents the karyotype, DNA content and meiotic behaviour of five species ofViciafrom Argentina (V. macrogramineaBurk.,V. gramineaSM.,V. epetiolarisBurk.,V. pampicolaBurk. andV. nanaVog.). All the species have the same chromosome number and karyotype formula (2n=14; 6m+4st+4t). Each species, however, displays a characteristic number and position of the nucleolar organizer region (NOR) and different sizes of the respective satellites, confirmed by Ag-NOR banding. Moreover, significant differences were found in the total chromosome volume (TCV) and DNA content of the species. Positive correlations between DNA content and TCV, and between DNA content and type of life cycle were also found. TCV and DNA content are lower inV. nana(annual) and higher inV. macrograminea(biennial–perennial). The material displayed marked karyotypic orthoselection, with similar karyotypes in all studied species, even when the overall chromosome size varied. Evolutionary changes in DNA amount are proportional to the relative length of each chromosome arm, maintaining karyotypic uniformity. Significant differences were found between the meiotic behaviour ofV. gramineaand that of the other species.V. gramineahas a lower frequency of ring bivalents and chiasmata per cell, and also has a lower interstitial chiasma frequency. In general, the results are congruent with the morphological data reported for these species.

Meiotic pairing in autohexaploid Lathyrus: a mathematical model

A mathematical meiotic pairing and chiasma model is described for a hexaploid. It estimates, on the basis of observations on chromosome associations at metaphase I of meiosis, the frequencies of initial hexavalent, quadrivalent and bivalent pairing, as well as the frequency of chiasmate association of the two chromosome arms, both after multivalent and after bivalent pairing. These estimates use all nine degrees of freedom available (10 configuration types distinguished). Although potential complications like multiple pairing partner exchange and interstitial chiasma formation are not accommodated by the model, a very close fit is found between the configuration frequencies observed in Lathyrus palustris (2n = 6x = 42) and those reconstructed on the basis of the model. This does not necessarily imply that the model fully represents biological reality, but at least it shows that the arms are significantly different in respect of chiasma formation and that chiasma frequencies may differ between multivalent and bivalent pairing. The hexavalent pairing frequency, as estimated by correcting metaphase frequencies for failure of chiasma formation, was only slightly lower than theoretically expected for an autohexaploid. However, the pairing of the six homologous chromosomes as a quadrivalent + a bivalent was considerably lower than with random pairing. The frequency of pairing as three bivalents was correspondingly higher.

Supernumerary heterochromatin, chiasma conditions and abnormal sperm formation inDichroplus elongatus(Orthoptera): intra and interpopulation analysis

SUMMARYParallel polymorphisms for B chromosome and supernumerary segments on M6, S9 and S10 pairs have been detected in two Argentinian populations of Dichroplus elongatus, Raco (RA) and Tafi Viejo (TV) (Tucuman Province). All these polymorphisms seem to be temporally stable. In all samples of TV, it was observed a tendency towards an excess of the B + SS6 class. SS6 produces a total restriction of chiasma formation in proximal position in the involved pair in heterozygous and homozygous males. In TV a consistent dose effect of B chromosome on interstitial chiasma (Xi) frequency was detected in males. The Xi frequency vs. number of supernumeraries plot fits to a linear regression. In both populations, the individuals with B's showed a significant increase in macro- and microspermatid production. This effect is more marked at RA. The cytophotometric analysis of DNA content revealed that in both types of males, B and non B carriers, most macrospermatids are diploid suggesting a probably origin by cytokinesi...

Identification of a homeologous chromosome pair by in situ DNA hybridization to ribosomal RNA loci in meiotic chromosomes of cotton (Gossypium hirsutum)

In situ DNA hybridization with 18S-28S and 5S ribosomal DNA probes was used to map 18S-28S nucleolar organizers and tandem 5S repeats to meiotic chromosomes of cotton (Gossypium hirsutum L.). Mapping was performed by correlating hybridization sites to particular positions in translocation quadrivalents. Arm assignment required translocation quadrivalents with at least one interstitial chiasma and sufficient distance between the hybridization site and the centromere. We had previously localized a major 18S-28S site to the short arm of chromosome 9; here we mapped two additional major 18S-28S sites to the short arm of chromosome 16 and the left arm of chromosome 23. We also identified and mapped a minor 18S-28S site to the short arm of chromosome 7. Two 5S sites of unequal size were identified, the larger one near the centromere of chromosome 9 and the smaller one near the centromere of chromosome 23. Synteny of 5S and 18S-28S sites indicated homeology of chromosomes 9 and 23, while positions of the other two 18S-28S sites supplement genetic evidence that chromosomes 7 and 16 are homeologous.

Analysis of chiasma frequency and first meiotic segregation in a human male reciprocal translocation heterozygote, t(1;11) (p36.3;q13.1), using fluorescence in situ hybridisation

In this study we have used a testicular biopsy from a human male with a 46,XY,t(1;11)(p36.3;q13.1) karyotype. Fluorescence in situ hybridisation with whole chromosome libraries and paracentromeric probes were applied to identify normal and derived chromosomes 1 and 11 in both first metaphase (MI) and second metaphase (MII) cells. The chiasma frequency distribution was established in the quadrivalent. A large proportion of MI cells was found to have at least one interstitial chiasma, resulting at MII in dimorphic chromosomes bearing one normal and one translocated chromatid. Alternate, adjacent I, adjacent II, and 3:1 products were all identified at MII. More than half of the cells analysed could not be assigned to a single segregation category because of the presence of interstitial chiasmata. Such MII cells could have arisen from either alternate or adjacent I segregation. We also calculated the proportion of sperm expected to be normal, balanced, and unbalanced. The latter data are in agreement with the results reported by Spriggs et al. (1992), who karyotyped sperm from the same individual.

B-chromosomes ofTrimerotropis pallidipennis(Oedipodinae, Acrididae): new effects on chiasma conditions

SUMMARY- Southamerican populations of Trimerotropis pallidipennis are polymor­ phic for both pericentric inversions and supernumerary chromosomes (MESA 1971; V AID eta!. 1979; GoN-1 et al. 1985; CoNFALONIERI 1988; CoNFALONIERI and CoLOMBO 1989). In the present paper, ten populations of this species distributed over several provinces of Argentina were analized with respect to chiasma conditions. Three populations showed a negative correlation between total and interstitial chiasma frequencies, but it was only apparent when B-carriers were excluded from the sample. This fact suggested that B-chromosomes would be interferring with the genetic control of chiasma formation. Besides, in two populations a significant drop of total chiasma frequencies from B-carriers with respect to non B-carriers was evident. Both modifica­ tions of chiasma conditions apparently influenced by the supernumerary chromosome have been rarely found among the Orthoptera. Interpopulational differences with respect to this effects are briefly discussed.

Multiple chromosome interchanges involving whole complement in Sesbania aculeata Poir.

Interchange complex involving full complement of chromosomes was induced in Sesbania aculeata Poir. through gamma irradiation. The association of twelve chromosomes was realized in 11.1 per cent cells in M3 generation and rest of the cells were characterized by break up of the complex into smaller units of different sizes and shapes. Low frequency of complete association, together with interstitial chiasma formation and lower terminalization coefficient of interchange heterozygotes indicated the small size of interchange segments.Predominant non-disjunctional orientation, unequal disjunction and break up of large complex into small units led to high percentage of inviable gametes as indicated by low pollen stainability.

Cytogenetic Studies inSinipta DalmaniStål (Orthoptera: Acrididae). I. Effects of a Pericentric Inversion on Chiasma Conditions

SUMMARYTwo populations of Sinipta dalmani (2n = 23, XO ♂/XX ♀) from Entre Rios Province (Argentina) are polymorphic for a pericentric inversion in pair M4 The inverted segment occupies about 40% of the chromosome lenght and changes its morphology from acrocentric to submetacentric. The frequencies of karyomorphs in each population are fitted to the expected according Hardy Weinberg's equilibrium. The inversion does not alter the mean total chiasma frequency. However, there is a significant decrease in interstitial chiasma frequency in the homozygous condition. Such decrement may be explained by a redistribution of chiasmata in medium sized bivalents. In the pair M4 the inversion precludes the crossing over within its limits in the heterozygous condition and increases terminal chiasma frequency in homozygosis. In the rest of the medium pairs, there is a significant decrease in interstitial chiasma frequency and a significant increase in proximal and distal chiasma frequencies in both heterozygous and homoz...

Studies on chiasma distribution and chiasma movement in Zoniopoda tarsata (Orthoptera: Romaleidae)

A sample of 27 males of Zoniopoda tarsata from Argentina was studied cytologically. The three largest autosomal pairs and the X were characterized by the presence of interstitial C-bands. Chiasma position relative to the bands was analyzed at diplotene and diakinesis. The frequency of interstitial, terminal and total chiasmata per cell was studied for the whole autosomal bivalent set, analysing the variations between stages and among individuals. The comparison of interstitial chiasma frequencies between stages and among individuals and the study of chiasma position relative to the bands in pairs 1, 2 and 3 indicated that chiasma distribution varied from diplotene to diakinesis. Therefore, terminalization does exist in this species and the movement may occur towards the centromere. The frequency of terminal associations at diplotene showed a high negative correlation (r=-0.89; p<10-5) with the number of interstitial chiasmata. This correlation would not be expected if the two kinds of association were produced by different (independent) mechanisms. Consequently, terminal associations were considered genuine chiasmata. The correlation between interstitial and total chiasmata was very much lower then the former (r=0.39; p=0.04). This fact, besides the relatively low variation for chiasma number, observed among individuals suggests that in this species the number of interestitial chiasmata, which are the most important in controlling the genetical recombination, is mainly regulated by changes in chiasma distribution, while variations in total chiasma frequency are of much lower magnitude.

The chromosomes of Micromys minutus (Rodentia, Murinae). II. Pairing pattern of X and Y chromosomes in meiotic prophase.

Both light and electron microscopy were used to study the pairing behavior of the sex chromosomes of the harvest mouse, Micromys minutus, in surface-spread pachytene spermatocytes. The XY pairing pattern is very exceptional in that the site of synaptic initiation is located interstitially in the short arms of the X and the Y, next to their centromeric regions. From this tiny euchromatic site, synapsis proceeds unidirectionally along the homologous heterochromatic short arms of the X and the Y toward the ends of the chromosomes. After pairing of the short arm is concluded, synapsis begins between the nonhomologous long arms of the X and the Y in the immediate vicinity of the centromeres and progresses unidirectionally toward the end of the long arm of the Y. A synaptic complex develops between the constitutive heterochromatin of the long arm of the Y and the euchromatin of the long arm of the X. Analysis of C-banded and distamycin A/DAPI-stained diakineses revealed a trefoil-like XY bivalent, which was interpreted to be the result of an interstitial chiasma occurring in the paired short arms of the X and the Y. A conspicuous, electron-dense body, about 1 micron in diameter, was found closely associated with the centromeres of the X and the Y in numerous pachytene spermatocytes. A review of the literature showed that comparable XY-associated bodies have been found in only eight other mammals to date.

The behavior and morphology of the X and Y chromosomes during prophase I in the Sitka deer mouse (Peromyscus sitkensis).

Surface-spread, silver-stained primary spermatocytes from individuals of the Sitka deer mouse (Peromyscus sitkensis) were analyzed by electron microscopy. Pairing of the X and Y chromosomes is initiated at early pachynema and is complete by mid pachynema. The pattern of sex chromosome pairing is unique in that it is initiated at an interstitial position, with subsequent synapsis proceeding in a unidirectional fashion towards the telomeres of the homologous segments. One-third the length of the X and two-thirds the length of the Y are involved in the synaptonemal complex of the sex bivalent. Various morphological complexities develop in the heteropycnotic (unpaired) segments as pachynema progresses, but desynapsis is not initiated until diplonema. Analysis of C-banded diakinetic nuclei indicated that sex chromosome pairing involves the heterochromatic short arm of the X and the long arm of the heterochromatic Y. An interstitial chiasma between the X and Y was observed in the majority of the diakinetic nuclei. The observation of a substantial pairing region and chiasma formation between the sex chromosomes of these deer mice is interpreted as indicating homology between the short arm of the X and the long arm of the Y.