Heteromorphic Bivalents Research Articles

TAMAÑO DEL GENOMA EN TRES ESPECIES DE Glandularia Y SUS HÍBRIDOS

In this work the relationship between genome size of Glandularia species and the meiotic configurations found in their hybrids are discussed. Glandularia incisa (Hook.) Tronc., growing in two localities of Corrientes and Córdoba provinces, Argentina, with different ecological conditions, showed inter-population variability of the 2C-value. The DNA content found in the Corrientes locality (2.41 pg) was higher than that obtained in the Córdoba locality (2.09 pg) which has more stressful environmental conditions than the former. These values are statistically different from those that were found in Glandularia pulchella (Sweet) Tronc. from Corrientes (1.43 pg) and in Glandularia perakii Cov. et Schn from Córdoba (1.47 pg). The DNA content of the diploid F1 hybrids, G. pulchella × G. incisa and G. perakii × G. incisa, differed statistically from the DNA content of the parental species, being intermediate between them. Differences in the frequency of pairing of homoeologous chromosomes were observed in the hybrids; these differences cannot be explained by differences in genome size since hybrids with similar DNA content differ significantly in their meiotic behavior. On the other hand, the differences in the DNA content between the parental species justify the presence of a high frequency of heteromorphic open and closed bivalents and univalents with different size in the hybrids. Key words: Intra-specific DNA content variability, homoeologous pairing, heteromorphic bivalents

An introgression on wheat chromosome 4DL in RL6077 (Thatcher*6/PI 250413) confers adult plant resistance to stripe rust and leaf rust (Lr67)

Adult plant resistance (APR) to leaf rust and stripe rust derived from the wheat (Triticum aestivum L.) line PI250413 was previously identified in RL6077 (=Thatcher*6/PI250413). The leaf rust resistance gene in RL6077 is phenotypically similar to Lr34 which is located on chromosome 7D. It was previously hypothesized that the gene in RL6077 could be Lr34 translocated to another chromosome. Hybrids between RL6077 and Thatcher and between RL6077 and 7DS and 7DL ditelocentric stocks were examined for first meiotic metaphase pairing. RL6077 formed chain quadrivalents and trivalents relative to Thatcher and Chinese Spring; however both 7D telocentrics paired only as heteromorphic bivalents and never with the multivalents. Thus, chromosome 7D is not involved in any translocation carried by RL6077. A genome-wide scan of SSR markers detected an introgression from chromosome 4D of PI250413 transferred to RL6077 through five cycles of backcrossing to Thatcher. Haplotype analysis of lines from crosses of Thatcher×RL6077 and RL6058 (Thatcher*6/PI58548)×RL6077 showed highly significant associations between introgressed markers (including SSR marker cfd71) and leaf rust resistance. In a separate RL6077-derived population, APR to stripe rust was also tightly linked with cfd71 on chromosome 4DL. An allele survey of linked SSR markers cfd71 and cfd23 on a set of 247 wheat lines from diverse origins indicated that these markers can be used to select for the donor segment in most wheat backgrounds. Comparison of RL6077 with Thatcher in field trials showed no effect of the APR gene on important agronomic or quality traits. Since no other known Lr genes exist on chromosome 4DL, the APR gene in RL6077 has been assigned the name Lr67.

Cytogenetics of three species of scorpions of the genus Brachistosternus from Argentina (Scorpiones: Bothriuridae)

Meiotic studies on three phylogenetically distant species of the genus Brachistosternus Pocock from Argentina were conducted. One species is from the subgenus Ministernus Francke 1985, B. ferrugineus Thorell 1876, and two species are from the subgenus Brachistosternus Pocock 1893, B. montanus Roig-Alsina 1977 (Andean species group), and morphologically different populations of B. pentheri Mello-Leitão 1931 (plains species group). All species showed achiasmatic meiosis, absence of heteromorphic bivalents, and bibrachial and monobrachial chromosomes of different sizes. Males of Brachistosternus ferrugineus, B. montanus, and one population of B. pentheri have 2n = 46. Males of the typical populations of B. pentheri have 2n = 42. These results suggest that B. pentheri may comprise two species.

Natural hybrids in Senecio (Asteraceae): new records from Argentina

Hybridisation is very common within Senecio. It is considered a fundamental mechanism of speciation and also a major feature causing systematic complexity. Morphological features are the preliminary characteristics used to identify hybrids; however this approach is usually not sufficient when this is the case chromosome studies represent a valuable tool for reinforcing the observations. Two natural homoploid interspecific hybrids are reported here: Senecio × lulioi M. G. López et Xifreda (S. brasiliensis (Spreng.) Less. × S. grisebachii Baker) and Senecio × piepaloensis M. G. López et Xifreda (S. ragonesei Cabrera × S. sectilis Griseb.). Classic cytogenetic analysis and pollen stainability were performed to evaluate hybrid origins and to designate the putative parents. S. × lulioi presented a low frequency of quadrivalents and heteromorphic bivalents, both strong evidences of hybridisation between closely related species. S. × piepaloensis exhibited a high frequency of meiotic irregularities, and reduced pollen stainability, certain characteristics of hybrids. This analysis is discussed in relation to the role of hybridisation in the evolution of the genus, and its systematic significance.

Interchanges in popcorn (Zea mays L.) involving the nucleolus organizer chromosome

The analysis of microsporogenesis in endogamous plants of popcorn (S5 to S7) showed several and distinct interchanges which involve the nucleolus organizer (chromosome 6). The detection of cells with interchanges was facilitated by the presence of two nucleoli of different sizes in contrast to normal ones with a single big nucleolus. Interchange points do not always seem to be at the same place. Whereas in several situations the interchange point clearly involved more than two chromosome pairs, a simple terminal translocation seemed to occur in others. During diplotene, a cross-shaped configuration connected with the nucleoli was observed in some meiocytes. Some heteromorphic bivalents were found during diakinesis, after which meiosis progressed normally to the end and gave rise to apparently normal tetrads with one normal nucleolus in each microspore. Tests of pollen viability in fixed pollen grains showed 100% stainability in normal and in affected plants. This is the first report on chromosome interchanges in popcorn.

Damaging Effect of Antibiotics on the Structure of Synaptonemal Complexes of Meiotic Mouse Chromosomes

The structure of synaptonemal complexes (SCs) of chromosomes of mouse primary spermatocytes were studied using electron microscopy on days 1, 10, and 36 after the completion of per os administration of drugs belonging to three groups of antibiotics: tetracyclins, macrolides, and fluoroquinolones. The antibiotics were administered to mice during ten days. At the substages of early and middle pachytene, heteromorphic SC bivalents and fragments of chromosome-core elements were detected in spermatocytes at all times studied after the administration of the antibiotics of three groups. As cells passed through the period from early to middle pachytene, the number of cells containing heteromorphic SC bivalents and the fragments of axial cores gradually decreased, which could be an indication of selection of cells with chromosomal aberrations. A high level of associations between the X chromosome and autosome bivalents (including heteromorphic ones) also favors this suggestion. A gradual decrease in the number of chromosomal aberrations was detected, as time elapsed from the completion of antibiotics administration. The study of sperm obtained from epididymises of males did not reveal significant differences in both morphology and motility of sperm between males of the control and experimental groups.

Cytological and genical differentiation between cytotypes ofEcheandia nana(Anthericaceae)

The analysis of 9 populations of Echeandia nana showed all to be dip- loid, with 2n = 16, n - 8 (x = 8). The analyzed populations displayed two cyto- types. Cytotype A — 10m+6sm, having two pairs of chromosomes with a satellite, was observed in four populations from the eastern flanks of the Pachuca mountain range. The five remaining populations from the western flank of the Pachuca and Sierra Nevada mountain ranges (Mexico) showed cytotype B = 6m+8sm+2st, having one pair of chromosomes with a satellite. The analysis of meiosis revealed heterozygotic exchanges. Analysis of MI showed three heteromorphic bivalents in cytotypes A and B. Analysis of AI showed sub-chromatid aberrations, (side arm bridges = SAB), which were more frequent in cytotype B (39.88-44.84%) than in cytotype A (2.86- 31-53%). Cells with two bridges (SAB aberrations) were observed in cytotype B. The intraspecific cytological and genical differentiation of cytotypes A and B is probably the result of geographical isolation between populations of E. nana. This suggests that this species is undergoing through a major process of genomic differentiation involving heterozygotic chromosomal rearrangements; which favors a process of speciation between both cytotypes without the occurrence of significant morphological changes. This cytological and genical differentiation between cytotypes A and B was evident in the significant differences of the low number of fruits and viable seeds produced after cross-pollination among cytotypes (AxB: 0-16 fruits; 0-448 abortive seeds), relative to the larger values recorded after cross- pollination within cytotypes (AxA: 4-48; 152-1824) (BxB: 3-89; 14-3382).

Studies of synaptonemal complexes in farm mammals--a review.

For the last 10 years extensive studies of synaptonemal complexes in farm mammals were carried out. In this article a survey of the obtained results is presented. Studies on synaptic behavior of chromosomes at pachytene substage of meiotic prophase I in carriers of centric fusions (cattle, sheep, goat, pig, and blue fox) showed that the trivalent is rarely involved in an association with the sex bivalent. In carriers of reciprocal translocations (pig and cattle) a wide range of synaptic configurations were found. Besides the expected cross-shaped quadrivalent, the following configurations were observed: open quadrivalent, trivalent plus univalent, and two heteromorphic bivalents. The latter configurations were quite frequently involved in the association with XY bivalent. Studies of synaptonemal complexes in interspecies hybrids (cattle x zebu, river x swamp buffalo, blue fox x silver fox) revealed a wide range of synaptic disturbances. Cases of pericentric inversions, aneuploidy, and chromosomal polymorphisms are also reviewed. This survey shows that synaptonemal complex analysis is a very useful tool in studies on causes of altered fertility in carriers of abnormal chromosome complement.

Chiasma redistribution in the presence of different sized supernumerary segments in a grasshopper: dependence on nonhomologous synapsis

Eight different sized supernumerary segments located at distal ends of the long arms of chromosomes M4, M5, M6, and S8 of the grasshopper Stenobothrus festivus were studied in males with regard to the synaptic process and chiasma distribution in the bivalents that carry them. The M4, M5, and M6 bivalents heterozygous for extra segments were always monochiasmate, in contrast to their bichiasmate condition observed in basic homozygotes. Furthermore, the presence of any of these extra segments led to chiasma redistribution in the carrier bivalents, so that such chiasmata were formed preferentially further away from the extra segment. The intensity of this effect is dependent on the size of the segment. Not all heteromorphic bivalents exhibited synaptonemal complexes with equalized axes at pachytene, but there was always a variable proportion of heterosynapsis around the distal ends of the long arms that was dependent on both the size of the segment and the size of the carrier chromosome. It is proposed that the absence of chiasmata in nonhomologous synapsed regions is responsible for the results obtained. Length measurements of the different extra segments and their carrier chromosomes between pachytene and diplotene indicated that synaptonemal complex is underrepresented in supernumerary heterochromatin.

Meiosis in North American and Asian Races of Gypsy Moth (Lepidoptera: Lymantriidae) and Their Hybrids

We have examined the meiosis of North American and Asian races of Lymantria dispar (L.) and their hybrids. The 31 prophase bivalents differ in size but bear few physical characteristics that allow a karyotype. Gypsy moth females are the heterogametic sex, and we suggest possible elements of the ZW pair. The meiotic prophases are achiasmate, but the chromosomes are tightly synapsed throughout prophase. A diffuse stage occurs, during which some chromosomes bear chromomeres. Male prophases are chiasmate. Meiosis of interracial hybrid females are normal and do not show evidence of heteromorphic bivalents, inversions, or translocations. Division anomalies are prevalent in the males, most anaphase figures representing unbalanced segregations. These observations are discussed in relation to efforts to characterize the gypsy broth genome.

Meiosis in carriers of heteromorphic bivalents: sex differences and implications for male fertility.

Mice that are double heterozygous for the semi-identical T(1;13)70H and T(1;13)1Wa reciprocal translocations display a great variation in male fertility. The synaptic behaviour of the different translocation chromosomes of adult males was studied in relation to this parameter. Juvenile males and embryonic females (16 and 18 days old) were included for comparison. In agreement with the minor differences In the translocation breakpoint positions, two differently sized heteromorphic bivalents are formed in meiotic prophase of both sexes (a quadrivalent was never encountered). Synaptonemal complex (SC) configurations of both bivalents in either sex are characterized by a high degree of non-homologous synapsis at zygotene-early pachytene. The rate of synaptic adjustment during pachytene is dependent on the size of the heteromorphic bivalent and varies between the sexes. Differences in SC configuration and morphology of the small heteromorphic bivalent in particular exist between the sexes and between animals. In males, this correlates with different degrees of fertility. Normal SC morphology in a fully synapsed small heteromorphic bivalent is an important determinant of successful meiosis and spermatogenesis. Moreover, aberrant synapsis favours the 'unsaturated pairing site' model as the primary cause for male sterility.

Synapsis in grasshopper bivalents heterozygous for centric shifts

Analysis of surface-spread synaptonemal complexes of zygotene and pachytene spermatocytes was carried out on centric-shift heterozygotes of grasshoppers. These rearrangements affected the M7 chromosome in Chorthippus vagans and the M6 and S8 chromosomes in Chorthippus apricarius. The shifts in the latter two chromosomes were also associated with C-heterochromatin variations between homologous chromosomes. Rearranged chromosomes proceeded directly to heterosynapsis without an apparent intervening homosynaptic phase in M7 bivalents of Ch. vagans and M6 bivalents of Ch. apricarius. In the latter case, axial equalization of the heterochromatin polymorphism was also achieved. On the other hand, asynapsis of the intercentromeric regions throughout pachytene was the rule in the centric shift involving the S8 chromosome of Ch. apricarius. In the three cases analysed, the production of unbalanced gametes in the heterozygotes is precluded either by the lack of chiasma formation in heterosynapsed rearranged segments or by the lack of pairing between such segments. Chiasmata were limited to the homologous regions of the heteromorphic bivalents.

The relationship between synapsis and chiasma distribution in grasshopper bivalents heterozygous for supernumerary segments

The relationship between chiasma distributions and pairing behaviour of grasshopper bivalent heterozygous for supernumerary segments has been analysed. The extra segments were located at the telomeric end of the long arm in chromosome S8 of Chorthippus jacobsi and at interstitial position in chromosomes S8 and M7 of Euchorthippus pulvinatus. In all cases these heteromorphic bivalents exhibited fully paired synaptonemal complexes with equalized axial lengths either at late zygotene or early pachytene, that is, heterosynapsis in regions near the extra segments is achieved at these meiotic stages. It is proposed that the absence of chiasmata in those regions in which nonhomologous pairing took place, would lead to chiasma redistributions observed in these bivalents when compared with those observed in bivalents in which extra segments are lacking.

Synaptonemal complex analysis of a reciprocal translocation, rcp(20;24) (q17;q25), in a subfertile bull

A reciprocal translocation was identified in a subfertile artificial insemination bull. Somatic chromosome investigation of G-banded metaphases demonstrated a 60,XY,rcp(20;24)(q17;q25) karyotype for the carrier. Synaptonemal complex analysis of the translocation by electron microscopy revealed an irregular pairing behavior of the chromosome axes involved, which resulted in a variety of configurations at pachytene. Not only was the expected quadrivalent configuration present, but also a trivalent plus univalent and two heteromorphic bivalents. Most common was an incompletely or completely paired quadrivalent configuration, which was non-homologously paired. XY association with the multivalent was seen only rarely. Histological analysis of testicular tissue showed meiotic arrest in some tubules. However, the semen picture was normal.

Identification of ‘Amigo’ and ‘Kavkaz’ translocations in Ohio soft red winter wheats (Triticum aestivum L.)

One cultivar ('GR876') and two advanced Ohio soft red winter wheat lines ('OH413' and 'OH414'), with 'Kavkaz' in their pedigrees, were examined for the presence of the 'Kavkaz,' 1RS/1BL rye/wheat chromosome translocation. Another advanced line ('OH416'), with 'Amigo' in its pedigree, was examined for the presence of the 'Amigo,' 1RS/1AL translocation. Only two satellited chromosomes were observed in most mitotic root-tip cells from 'GR876,' 'OH413,' and 'OH414,' compared to four in most cells from 'OH416.' Heteromorphic bivalents were observed in most PMCs from hybrids produced by crossing 'GR876,' 'OH413,' and 'OH414' as females to 'Chinese Spring.' No heteromorphic bivalents were observed in PMCs from 'OH416' x 'Chinese Spring' hybrids. When 'GR876' and the Ohio lines were hybridized with 'Chinese Spring' dimonotelosomic-1B, telosomic trivalents, consisting of the short- and longarm telosomes paired with chromosome 1B, were only observed in PMCs from 43-chromosome hybrids involving 'OH416.' The long-arm telosome paired with the translocation chromosome, while the short-arm telosome remained unpaired in all other 43-chromosome hybrids. Separation of gliadin proteins from 'GR876' and the Ohio lines by PAGE revealed that secalin bands for 'GR876,' 'OH413,' and 'OH414,' migrated similarly to the secalins for 'Kavkaz.' Bands for 'OH416,' identified as possible secalins, migrated similarly to those for 'Amigo.' Cultivar 'GR876' and advanced Ohio soft red winter wheat lines 'OH413' and 'OH414' carry the 'Kavkaz' translocation, while 'OH416' carries the 'Amigo' translocation.

Synaptonemal Complex Analysis of Sex Chromosomes in Two Species of Sceloporus

Silver-stained synaptonemal complexes were analyzed to examine chromosomal pairing in two species of Sceloporus (S. graciosus and S. undulatus) that have indistinct sex chromosomes. Electron microscopic analyses revealed distinct length heteromorphism between the lateral elements of one of the largest microchromosomal synaptonemal complexes in each species. The morphology and behavior of the heteromorphic synaptonemal complex in S. graciosus and S. undulatus were congruous with those described for heteromorphic sex bivalents in other vertebrates and are hypothesized to represent synapsis of the sex chromosomes. Synaptic behavior of the heteromorphic bivalents was similar between species and differed from that of the homomorphic (autosomal) bivalents within each species. In both species, synapsis of the heteromorphic bivalent was characterized by the formation of a buckle in the synaptonemal complex at early to mid-pachynema. Synaptic adjustment was observed to result in equalization in length of the lateral elements.

Synaptonemal complex spreading in Allium cepa and Allium fistulosum. III. The F1 hybrid

Synaptonemal complexes (SCs) were analysed, by surface spreading, in the F1 hybrid between Allium cepa and Allium fistulosum. These closely related species have a similar karyomorphology, but the A. cepa genome has 27% more DNA than A. fistulosum. At metaphase I, in the hybrid, heteromorphic bivalents were observed, and the pollen mother cell chiasma frequency in the F1 hybrid was reduced when compared with the parents. Synapsis was incomplete and disturbed to some extent in all the prophase I nuclei observed. The reduced level of synapsis at late zygotene – mid pachytene in the hybrid (mean percent homoeologous synapsis is 60%) corresponds to the percent reduction in chiasma frequency. It is suggested that failure of synapsis is the main cause of reduction of chiasma frequency in the F1 hybrid. The chiasma distribution in the hybrid is most similar to that of the A. cepa parent. Although some proximal chiasmata were observed (10% of total), none of the bivalents observed had only proximally localised chiasmata as found in the A. fistulosum parent. In the F1 hybrid, synapsis of the centromeric regions of the bivalents was invariably disturbed throughout prophase I. It is suggested that there are major DNA differences in these regions preventing regular synapsis or progression of synapsis and possibly proximal chiasma formation. Nonhomoeologous synapsis occurred within axes, giving foldback SCs, and between axes, resulting in multivalent formation. It is proposed that disturbance of synapsis is due to DNA differences. There is no indication of any rearrangement or adjustment of synapsis during prophase I either to give perfectly synapsed bivalents or to resolve multivalents into bivalents.Key words: synaptonemal complex, Allium, incomplete pairing, chiasma, nonhomologous pairing, DNA content.

Synapsis in single and double heterozygotes for partially overlapping inversions in chromosome 1 of the house mouse

Electron microscopic (EM) analysis of synaptonemal complexes (SC) in single and double heterozygotes for the partially overlapping inversions In(1)1Icg, In(1)1Rk and In(1)12Rk in chromosome 1 of the house mouse reveals that synapsis and synaptic adjustment are dependent on the size and location of the inversions and interaction between the latter. Is(1)1Icg contains insertions of the inverted repeats Is(HSR;1C5)1Icg and Is(HSR;1D)2Icg and an inverted euchromatic region. Synaptic adjustment of the D-loops by shortening of the asynapsed segments of the lateral elements belonging to the insertions occurs at the late zytogene to early pachytene stage. Synaptic adjustment of the inversion loops takes place at early to late pachytene. A delay in adjustment was found in the double heterozygotes In(1)1Icg/In(1)1Rk and In(1)1Icg/In(1)12Rk. A correspondence between the lifespan of asynapsis in inverted regions and the probability of association of XY and heteromorphic bivalents was revealed.

Meiotic crossing-over between sites on opposite sides of the centromeres of homoeologues is frequent in hybrid Aloeaceae

During meiosis, long and short arms of acrocentric homoeologues pair and cross over in the centromere region in 95 (66.9%) of 142 hybrids of differing parentage in the monocotyledon family Aloeaceae. A characteristic configuration, the L–S bridge, is produced at anaphase I with frequencies ranging from <1 to 48% of pollen mother cells and in up to three bivalents per pollen mother cell. Too frequent to be due to inversion hybridity, L–S crossing-over most probably results from straight, noninverted pairing between nonhomologous proximal segments of the long and short chromosome arms following centromere mismatching in the heteromorphic bivalents. It is suggested that there are several lengths of DNA in different regions of homoeologous chromosomes, but perhaps concentrated around the centromere, that are sufficiently similar to recognize each other, pair, and cross over when brought together in a heteromorphic bivalent with mismatching of centromeres.Key words: Aloeaceae, hybrid, meiosis, nonhomologous pairing, crossing-over.

Double Insertion of Homogeneously Staining Regions in Chromosome 1 of Wild Mus <italic>musculus musculus</italic>: Effects on Chromosome Pairing and Recombination

An examination of the meiotic pattern of chromosome 1 isolated from a feral mouse population and containing a double insertion (Is) of homogeneously staining regions (HSRs) was carried out. The region delineated by the proximal breakpoint of Is(HSR;1C5) 1Icg and the distal breakpoint of Is(HSR;1E3)2Icg is desynapsed during the early pachytene stage and heterosynapsed at the midpachytene, as shown by electron microscopic analysis of synaptonemal complexes. The HSRs have no effect on the segregation of chromosome 1 in heterozygous mice. The lack of homosynapsis in the region under study causes chiasmata redistribution in heteromorphic bivalents. In normal males, single chiasmata are located in the medial part of the chromosome. In heterozygotes, this segment is heterosynapsed and unavailable for recombination. This leads to a significant decrease in the frequency of bivalents bearing single chiasmata. The total number of chiasmata per bivalent is much higher in heterozygous males than in normal ones. The recombination frequency between proximal markers fz and In also is higher in heterozygous animals. The increase in the total chiasma number in the heteromorphic bivalent is due to the addition of double chiasmata located mostly at precentromeric and pretelomeric regions of the chromosome.