Sex chromosomes and origin of males and sex mosaics of the parthenogenetic stick insect Carausius morosus Br.

The chromosome complements of sporadic males and masculinized females of the thelytokous phasmid Carausius morosus Br. could be analysed in spermatogonia and ovarian follicle cells. Masculinized females with ovaries, ovotestes or testes have the female chromosome number, i.e., 61 autosomes and three sex chromosomes. The sex chromosomes, one being longer than the other two, are metacentric. In one masculinized female with testes the three sex chromosomes were different, apparently through a reciprocal translocation. The masculinized females are considered to be intersexes (phenotypic sex determination). The chromosome complement of males differs from that of females by lacking either one of the sex chromosomes or only a segment of one of these chromosomes (genotypic sex determination). The deleted sex chromosomes appear as acrocentrics and may have arisen through a chiasma between a translocated segment in one sex chromosome and its untransposed homologous region in another sex chromosome. One apparently telocentric sex chromosome may have originated from centric fission together with loss of the other arm. The sex chromosomes are positively heteropycntoic in the psermatogonia, also in those of masculinized females. En bloc heterochromatinization of the sex chromosomes, which seems to be under the direct or indirect control of one or more sites on the sex chromosomes themselves, functions in sex determination. The sex determination does not give a decisive answer to the question whether di-, tri-, or tetraploidy is involved.     

Sex chromosome evolution in fish: the formation of the neo-Y chromosome in Eigenmannia (Gymnotiformes)

Chromosomes of a species of Eigenmannia presenting a X₁X₁X₂X₂:X₁X₂Y sex chromosome system, resulting from a Y-autosome Robertsonian translocation, were analyzed using the C-banding technique, chromomycin A₃ (CMA₃) and mithramycin (MM) staining and in situ digestion by the restriction endonuclease AluI. A comparison of the metacentric Y chromosome of males with the corresponding acrocentrics in females indicated that a C-band-positive, CMA₃/MM-fluorescent and AluI digestion-resistant region had been lost during the process of translocation, resulting in a diminution of heterochromatin in the males. It is hypothesized that the presence of a smaller amount of G+C-rich heterochromatin in the sex chromosomes of the heteromorphic sex when compared with the homomorphic sex may be associated with the sex determination mechanism in this species and may be a more widely occurring phenomenon in fish with differentiated sex chromosomes than was initially thought. Received: 1 April 1999; in revised form: 16 October 1999 / Accepted: 4 December 1999     

The Y- autosome ttanslocation of Callimico goeldii

Intraspecific chromosomal variability is common among New World primates. A polymorphism has been described among male Callimico goeldiiin which the Y chromosome is translocated to an autosome. Consequently, males may have a chromosome number of 47 or 48. We describe the results of karyotypic analyses on 40 captive male C. goeldii.Thirty- nine of them had a diploid chromosome number of 47, including the Y- autosome translocation. The remaining male had 48 chromosomes;however, he too carried the translocation along with two X chromosomes. The reported Y- chromosome translocation in Callimico goeldiiappears not to be a polymorphism but, instead, a feature characteristic of all males in the population.     

The C. elegans sex determination gene laf-1 encodes a putative DEAD-box RNA helicase

The Caenorhabditis elegans gene laf-1 is critical for both embryonic development and sex determination. Laf-1 is thought to promote male cell fates by negatively regulating expression of tra-2 in both hermaphrodites and males. We cloned laf-1 and established that it encodes a putative DEAD-box RNA helicase related to Saccharomyces cerevisiae Ded1p and Drosophila Vasa. Three sequenced laf-1 mutations are missense alleles affecting a small region of the protein in or near helicase motif III. We demonstrate that the phenotypes resulting from laf-1 mutations are due to loss or reduction of laf-1 function, and that both laf-1 and a related helicase vbh-1 function in germline sex determination. Laf-1 mRNA is expressed in both males and hermaphrodites and in both the germline and soma of hermaphrodites. It is expressed at all developmental stages and is most abundant in embryos. LAF-1 is predominantly, if not exclusively, cytoplasmic and colocalizes with PGL-1 in P granules of germline precursor cells. Previous results suggest that laf-1 functions to negatively regulate expression of the sex determination protein TRA-2, and we find that the abundance of TRA-2 is modestly elevated in laf-1/+ females. We discuss potential functions of LAF-1 as a helicase and its roles in sex determination.     

Genomic changes following the reversal of a Y chromosome to an autosome in Drosophila pseudoobscura

Robertsonian translocations resulting in fusions between sex chromosomes and autosomes shape karyotype evolution by creating new sex chromosomes from autosomes. These translocations can also reverse sex chromosomes back into autosomes, which is especially intriguing given the dramatic differences between autosomes and sex chromosomes. To study the genomic events following a Y chromosome reversal, we investigated an autosome‐Y translocation in Drosophila pseudoobscura. The ancestral Y chromosome fused to a small autosome (the dot chromosome) approximately 10–15 Mya. We used single molecule real‐time sequencing reads to assemble the D. pseudoobscura dot chromosome, including this Y‐to‐dot translocation. We find that the intervening sequence between the ancestral Y and the rest of the dot chromosome is only ～78 Kb and is not repeat‐dense, suggesting that the centromere now falls outside, rather than between, the fused chromosomes. The Y‐to‐dot region is 100 times smaller than the D. melanogaster Y chromosome, owing to changes in repeat landscape. However, we do not find a consistent reduction in intron sizes across the Y‐to‐dot region. Instead, deletions in intergenic regions and possibly a small ancestral Y chromosome size may explain the compact size of the Y‐to‐dot translocation.     

Sex determination without the Y Chromosome in two Japanese rodents Tokudaia osimensis osimensis and Tokudaia osimensis spp.

Both males and females of the species of spinous country-rats (Tokudaia osimensis osimensis, T. o. o., Rodentia: Muridae), which live on Amami Oshima Island, a southern Japanese island, have 25 chromosomes. Another species of spinous country-rats (Tokudaia osimensis spp., T. o. spp., which live on Tokunoshima Island 40 km south of Amami Oshima Island, also have an odd number of chromosomes, 45. Karyotypes of males and females by the G-band method were indistinguishable in both populations. The lesser number of chromosomes (25) of T. o. o. is likely to be a result of Robertsonian fusions of 45 chromosomes of T. o. spp. that seem to be the offspring of another spinous country-rat Tokudaia osimensis muenninki (T. o. m.), which live on Okinawa Island and have 44 chromosomes including the X and Y Chrs. The lengths of the non-paired, putative X-Chr of T. o. o. and T. o. spp. occupied roughly 3.2% and 1.7% of the total lengths, respectively, hinting at translocation or exchange of a part of the X Chr and thus in violation of Ohno's Law. Southern blot analysis with murine Sry as a probe indicated that these two animals do not have Sry. When Zfx from T. o. spp. was used as a probe, both males and females of T. o. o. and T. o. spp. showed two bands, suggesting possible translocation of Zfy from the Y Chr. Comparison of physical characteristics, constituents of chromosomes, and sex-determination methods of these three Tokudaia country-rat populations suggests that each is endemic to each island and constitutes an independent species. These specialized species would provide us with clues to elucidate the mechanisms of primary sex determination and karyotype evolution in mammals. Received: 2 June 2000 / Accepted: 24 August 2000     

A complex sex-chromosome system in the hare-wallaby Lagorchestes conspicillatus Gould

Among specimens of the spectacled hare-wallaby Lagorchestes conspicillatus Gould (Marsupialia, family Macropodidae) 4 males had 15 chromosomes and 2 females 16 chromosomes. The sex chromosomes are X₁X₁X₂X₂ in the female and X₁X₂Y in the male, the Y being metacentric and both X chromosomes are acrocentric. In about 96% of sperm mother cells at meiosis the sex chromosomes form a chain trivalent and in more than 99% of these this orients convergently so that the X₁ and X₂ move to the same pole. Evidence is presented that L. conspicillatus has evolved from a form with 22 chromosomes including a small X and a minute Y. Autoradiographic studies show that the proximal fifth of the X₁ chromosome replicates late. This is probably the ancestral X chromosome which has been translocated to an autosome. The fate of the original Y is obscure but an hypothesis is proposed that it forms the centromeric region of the Y. A single male had 14 chromosomes and was heterozygous for a translocation involving the centric fusion of two acrocentric autosomes. In about 30% of sperm mother cells the autosomal trivalent did not disjoin regularly but, despite this, all secondary spermatocytes observed at metaphase 2 had balanced complements of chromosomes. It is assumed that unbalanced secondary spermatocytes died before reaching metaphase.     

Causes and consequences of reciprocal translocations on sex chromosomes

Under XY sex determination, the Y chromosome is only inherited via males, whereas the X chromosome is predominantly found in females. Thus, it is favourable when alleles with high male fitness become associated with the Y chromosome and when alleles with high female fitness become associated with the X chromosome. These favourable associations can be strengthened through linkage. Rearrangements, such as inversions and sex chromosome–autosome fusions, can increase linkage and thereby become favoured (Charlesworth, 2017). In a From the Cover article in this issue of Molecular Ecology, Toups, Rodrigues, Perrin, and Kirkpatrick (2019) present the first genomic analysis of a sex chromosome reciprocal translocation, a particularly dramatic chromosomal rearrangement that modifies linkage with the sex chromosome. As a result of reciprocal translocation, one studied population of the common frog (Rana temporaria, Figure 1) displays a remarkable sex‐determining system in which there are two physically unlinked sex chromosomes that are exclusively cotransmitted (Figure 2a).     

Identifizierung der genetischen Geschlechtschromosomen bei der monogenen Schmeißfliege Chrysomya rufifacies (Calliphoridae,Diptera)

Previous investigations have shown the sex determination in the monogenic blowfly Chrysomya rufifacies to be controlled by a cytologically not discernible homogamety-heterogamety mechanism in the female. Female-producing (thelygenic) females are assumed to be heterozygous for a dominant female sex realizer (F′) with sex-predetermining properties, while male-producing (arrhenogenic) females as well as males are supposed to be homozygous for the recessive allele (f). In order to identify the genetic sex chromosomes of C. rufifacies among its five pairs of long euchromatic chromosomes (nos.1–5) plus one pair of small heterochromatic ones (no. 6), all chromosomes were marked by reciprocal translocations induced by X-ray treatment of adult males. The inheritance of thirteen different heterozygous translocations has been analyzed. All of the translocations (eleven) between two of the four longer chromosomes did not show sex-linked inheritance, thus demonstrating the autosomal character of the chromosomes nos 1, 2, 3 and 4. The same is true for the translocation T6 (2/6). Therefore the small heterochromatic chromosome no. 6, corresponding to the morphologically differentiated sex chromosomes within the amphogenic calliphorid species, remains without sex determining function in the monogenic fly. This could be confirmed by the analysis of monosomic (monosomy-6) and trisomic (trisomy-6) individuals, which resulted from meiotic non-disjunction in T6/+ translocation heterozygotes. Contrary to these translocations, the heterozygous 5/2 translocation (T14) exhibited sex-linked inheritance: There was but a very low frequency (0,76 per cent) of recombinants resulting from crossing-over between F′/f and the translocation breakage point in thelygenic F₁ T14/+ females. The sex-linked inheritance of T14 was confirmed by the progeny of a thelygenic F₁ T14/+ female crossed to a homozygous T14/T14 translocation male. Among the offspring of that F₁ T14/+ female, which had received the translocation from its father, all of the F₂ T14/+ females were thelygenic compared to their arrhenogenic T14/T14 sisters. These results prove that the chromosomes of pair no. 5 genetically act as X′X-XX sex chromosomes in C. rufifacies.     

Ultrastructure,meiotic behavior,and evolution of sex chromosomes of the genus Ellobius

The whole-mount SC preparations from males of three species of the genus Ellobius (Ellobius fuscocapillus, Ellobius lutescens), and Ellobius tancrei were studied by electron microscopy. In the males of Ellobius fuscocapillus, behavioral peculiarities of the sex bivalent (viz. the normal male heterozygosity) are characterized by early complete desynapsis of sex chromosomes (X, Y), occurring at late pachytene-early diplotene. The karyotype of species Ellobius lutescens is unique for mammals. In both sexes it is characterized by an odd number of chromosomes (2n=17). At prophase I the unpaired chromosome 9 is not involved in synapsis with other chromosomes and forms a sex body at the end of pachytene.The complete Robertsonian fan has been described for superspecies Ellobius tancrei. As shown on the basis of G-band patterns the male and female sex chromosomes are cytologically indistinguishable.Analysis of whole-mount SC preparations revealed the formation of a closed sex SC bivalent and showed some morphological differences in the axes of sex chromosomes at meiotic prophase I. A number of assumptions are made about the relationship between the behavior of sex chromosomes, their evolution and the sex determination system in the studied species of genus Ellobius.

---

     

In vitro rearing ofBracon mellitor andCatolaccus grandis with artificial diets devoid of insect components

Bracon mellitor Say (Hymenoptera: Braconidae), andCatolaccus grandis Burks (Hymenoptera: Pteromalidae), ectoparasitoids of the boll weevil,Anthonomus grandis Boheman, were rearedin vitro for the first time on artificial diets devoid of insect components. The duration of the life cycles from egg to adult when these parasitoids were rearedin vivo on boll weevil larvae, on semi-artificial diets retained with cotton fabric pads, or on diets with low agar concentrations (0.7%) were ca. 15.5, 16.5, and 15.0 days, forB. mellitor and 16.0, 17.5, and 15.5 days, respectively, forC. grandis. Percent adult emergence obtained with the same treatments was 58, 25, and 65 percent, forB. mellitor and 50, 28, and 60 percent, respectively, forC. grandis. Adult morphological characteristics and mating appeared normal. Male to female sex ratios were ca. 45∶55 for both species.     

Ultrastructural characterization of the sex chromosomes during spermatogenesis of spiders having holocentric chromosomes and a long diffuse stage

An ultrastructural study has been made of spermatogenesis in two species of primitive spiders having holocentric chromosomes (Dysdera crocata, XO and Segestria florentia X₁X₂O). Analysis of the meiotic prophase shows a scarcity or absence of typical leptotene to pachytene stages. Only in D. crocata have synaptonemal complex (SC) remnants been seen, and these occurred in nuclei with an extreme chromatin decondensation. In both species typical early prophase stages have been replaced by nuclei lacking SC and with their chromatin almost completely decondensed, constituting a long and well-defined diffuse stage. Only nucleoli and the condensed sex chromosomes can be identified. — In S. florentina paired non-homologous sex chromosomes lack a junction lamina and thus clearly differ from the sex chromosomes of more evolved spiders with an X₁X₂O male sex determination mechanism. In the same species, sex chromosomes can be recognized during metaphase I due to their special structural details, while in D. crocata the X chromosome is not distinguishable from the autosomes at this stage. — The diffuse stage and particularly the structural characteristics of the sex chromosomes during meiotic prophase are reviewed and discussed in relation to the meiotic process in other arachnid groups.     

Chromosome polymorphisms close to the cm-ADE1 locus of Candida maltosa

The imperfect yeast Candida maltosa has an ill-defined genetic constitution; it is nominally diploid, but probably highly aneuploid, in nature. We report on polymorphisms specifically affecting those chromosomes which bear the cm-ADE1 gene. This gene encodes phosphoribosylaminoimidazole-succinocarboxamide synthetase, an enzyme in the adenine biosynthetic pathway. By electrophoretic karyotype analysis, three differently sized chromosomes were demonstrated to carry cm-ADE; the size (but not the number) of these chromosomes was also found to vary, both between strains and during the mitotic growth of a single strain. Four different alleles of cm-ADE1 have been cloned and sequenced from one prototrophic strain. DNA sequence divergence between these different alleles is as high as 8%, with the greatest divergence being found in the upstream region. Mitotic recombination events that led to changes in the karyotype were followed by using cm-ADE1 DNA as an hybridization probe. A recombination hot-spot in the neighbourhood of the gene appears to be responsible for the instability of the chromosomes on which it resides.     

HOMOLOGOUS SEX CHROMOSOMES IN THREE DEEPLY DIVERGENT ANURAN SPECIES

Comparative genomic studies are revealing that, in sharp contrast with the strong stability found in birds and mammals, sex determination mechanisms are surprisingly labile in cold‐blooded vertebrates, with frequent transitions between different pairs of sex chromosomes. It was recently suggested that, in context of this high turnover, some chromosome pairs might be more likely than others to be co‐opted as sex chromosomes. Empirical support, however, is still very limited. Here we show that sex‐linked markers from three highly divergent groups of anurans map to Xenopus tropicalis scaffold 1, a large part of which is homologous to the avian sex chromosome. Accordingly, the bird sex determination gene DMRT1, known to play a key role in sex differentiation across many animal lineages, is sex linked in all three groups. Our data provide strong support for the idea that some chromosome pairs are more likely than others to be co‐opted as sex chromosomes because they harbor key genes from the sex determination pathway.     

Sex without sex chromosomes: genetic architecture of multiple loci independently segregating to determine sex ratios in the copepod Tigriopus californicus

Sex‐determining systems are remarkably diverse and may evolve rapidly. Polygenic sex‐determination systems are predicted to be transient and evolutionarily unstable, yet examples have been reported across a range of taxa. Here, we provide the first direct evidence of polygenic sex determination in Tigriopus californicus, a harpacticoid copepod with no heteromorphic sex chromosomes. Using genetically distinct inbred lines selected for male‐ and female‐biased clutches, we generated a genetic map with 39 SNPs across 12 chromosomes. Quantitative trait locus mapping of sex ratio phenotype (the proportion of male offspring produced by an F2 female) in four F2 families revealed six independently segregating quantitative trait loci on five separate chromosomes, explaining 19% of the variation in sex ratios. The sex ratio phenotype varied among loci across chromosomes in both direction and magnitude, with the strongest phenotypic effects on chromosome 10 moderated to some degree by loci on four other chromosomes. For a given locus, sex ratio phenotype varied in magnitude for individuals derived from different dam lines. These data, together with the environmental factors known to contribute to sex determination, characterize the underlying complexity and potential lability of sex determination, and confirm the polygenic architecture of sex determination in T. californicus.     

Chromosomal location of 45S rDNA and dfr gene in Citrus sinensis

Dihydroflavonol 4-reductase (DFR) is a key enzyme in the anthocyanin biosynthesis. In this study, the localization of 45S rDNA and dfr gene (named as CsDFR-bo) of the blood orange (Citrus sinensis Osbeck cv. Ruby) on chromosomes was investigated by fluorescence in situ hybridization (FISH). A karyotype of C. sinensis was reconstructed based on the length of mitotic metaphase chromosomes. The 45S rDNAs were localized on chromosomes 2p and 7q. The detection ratios of 45S rDNA in the two chromosomes were 69.5 and 77.3 %, respectively. The CsDFR-bo was proved to be a single-copy gene and localized on chromosome 3p. The detection ratio of CsDFR-bo was 8.2 %.     

The murine Rb(6.16) translocation: evidence for sperm selection and a modulating effect of aging

Summary The segregation products of the Rb(6.16) translocation were studied at first cleavage metaphase. Male mice heterozygous for the translocation mated at 3- and 14-day intervals to superovulated random-bred ICR females. Chromosome preparations of the recovered one-cell embryos were sequentially G- and C-banded and male and female complements analyzed cytogenetically. Of the 309 zygotes analyzed from both mating groups, no unbalanced segregants of the translocation were observed. In the 3-day group there was a highly significant difference (P<0.001) from the expected 1:1 ratio between sperm with normal (70.5%) and balanced segregants (26.2%) of alternate segregation. In the 14-day group the level of significance for the difference was ten times lower (P<0.01) as normal segregants were observed in 56.4% of the sperm and balanced ones in 36.5%. A comparison of the two groups using a 2×2 contingency table showed that segregant type was related to mating frequency (P<0.05). There was a highly significant distortion (P<0.01) of the sex ratio, with 178 Y-bearing and 131 X-bearing sperm in the combined populations. When sex ratio was analyzed according to mating intervals, the distortion was significant (P<0.01) only for the 3-day group. An analysis of the sex ratio according to the segregant type showed no significant deviation from 1:1 for type 1 segregants, which had normal chromosomes, while type 2 segregants, with the translocation, had a deficiency of X-bearing sperm. This deficiency was significant (P<0.05) only for the 3-day population. Analysis of meiotic preparations showed no association between the translocation trivalent and the X-Y bivalent. Our results, obtained under physiological conditions, provide definitive evidence for sperm selection and support previous findings that aging of sperm can modify the effect of selection.