X1X1X2X2/X1X2Y sex chromosome systems in the Neotropical Gymnotiformes electric fish of the genus Brachyhypopomus

Several types of sex chromosome systems have been recorded among Gymnotiformes, including male and female heterogamety, simple and multiple sex chromosomes, and different mechanisms of origin and evolution. The X₁X₁X₂X₂/X₁X₂Y systems identified in three species of this order are considered homoplasic for the group. In the genus Brachyhypopomus, only B. gauderio presented this type of system. Herein we describe the karyotypes of Brachyhypopomus pinnicaudatus and B. n. sp. FLAV, which have an X₁X₁X₂X₂/X₁X₂Y sex chromosome system that evolved via fusion between an autosome and the Y chromosome. The morphology of the chromosomes and the meiotic pairing suggest that the sex chromosomes of B. gauderio and B. pinnicaudatus have a common origin, whereas in B . n. sp. FLAV the sex chromosome system evolved independently. However, we cannot discard the possibility of common origin followed by distinct processes of differentiation. The identification of two new karyotypes with an X₁X₁X₂X₂/X₁X₂Y sex chromosome system in Gymnotiformes makes it the most common among the karyotyped species of the group. Comparisons of these karyotypes and the evolutionary history of the taxa indicate independent origins for their sex chromosomes systems. The recurrent emergence of the X₁X₁X₂X₂/X₁X₂Y system may represent sex chromosomes turnover events in Gymnotiformes.     

The role of the Robertsonian rearrangements in the origin of the XX/XY1Y2 sex chromosome system and in the chromosomal differentiation in Harttia species (Siluriformes, Loricariidae)

Harttia is a genus of the subfamily Loricariinae that posses a broad chromosomal variation. In addition to interspecific karyotype diversity within this group, a multiple sex chromosome system, XX/XY₁Y₂, has been described for Harttia carvalhoi. Thus, this study aimed to determine the role of chromosomal rearrangements in karyotype differentiation in Harttia by classical and molecular cytogenetic procedures. The results show that Robertsonian rearrangements have a prominent role in the chromosomal diversification of the species analysed, which initially leads to hypothesize a diploid number reduction in Harttia torrenticola and H. carvalhoi. The metacentric chromosome 1, shared between H. torrenticola and H. carvalhoi, could have originated from centric fusions from the ancestral karyotype. A centric fission event associated with the first metacentric pair allowed for the origination of a multiple sex chromosome system XX/XY₁Y₂, specific to H. carvalhoi. This study highlights the relevance of Robertsonian rearrangements in karyotypic differentiation of the species studied and demonstrates that the occurrence of a centric fission, as opposed to a previously hypothesised chromosome fusion, is directly implicated in the origin of the sex chromosome system of H. carvalhoi.     

Multiple sex chromosome system of X1X1X2X2/X1X2)Y type in lutjanid fish, Lutjanus quinquelineatus (Perciformes)

The karyotype and other chromosomal markers as revealed by C-banding and Ag-staining were studied in Lutjanus quinquelineatus and L. kasmira (Lutjanidae, Perciformes). While in latter species, the karyotype was invariably composed of 48 acrocentric chromosomes in both sexes, in L. quinquelineatus the female karyotype had exclusively 48 acrocentric chromosomes (2n = 48) but that of the male consisted of one large metacentric and 46 acrocentric chromosomes (2n = 47). The chromosomes in the first meiotic division in males showed 22 bivalents and one trivalent, which was formed by an end-to-end association and a chiasmatic association. Multiple sex chromosome system of X₁X₁X₂X₂/X₁X₂Y type resulting from single Robertsonian fusion between the original Y chromosome and an autosome was hypothesized to produce neo-Y sex chromosome. The multiple sex chromosome system of L. quinquelineatus appears to be at the early stage of the differentiation. The positive C-banded heterochromatin was situated exclusively in centromeric regions of all chromosomes in both species. Similarly, nucleolus organizer region sites were identified in the pericentromeric region of one middle-sized pair of chromosomes in both species. The cellular DNA contents were the same (3.3 pg) between the sexes and among this species and related species.     

Chromosomal rearrangements and the first indication of an ♀X1X1X2X2/♂X1X2Y sex chromosome system in Rineloricaria fishes (Teleostei: Siluriformes)

Rineloricaria is the most diverse genus within the freshwater fish subfamily Loricariinae, and it is widely distributed in the Neotropical region. Despite limited cytogenetic data, records from southern and south-eastern Brazil suggest a high rate of chromosomal rearrangements in this genus, mirrored in remarkable inter- and intraspecific karyotype variability. In the present work, we investigated the karyotype features of Rineloricaria teffeana, an endemic representative from northern Brazil, using both conventional and molecular cytogenetic techniques. We revealed different diploid chromosome numbers (2n) between sexes (33♂/34♀), which suggests the presence of an ♀X₁X₁X₂X₂/♂X₁X₂Y multiple sex chromosome system. The male-limited Y chromosome was the largest and the only biarmed element in the karyotype, implying Y-autosome fusion as the most probable mechanism behind its origination. C-banding revealed low amounts of constitutive heterochromatin, mostly confined to the (peri)centromeric regions of most chromosomes (including the X₂ and the Y) but also occupying the distal regions of a few chromosomal pairs. The chromosomal localization of the 18S ribosomal DNA (rDNA) clusters revealed a single site on chromosome pair 4, which was adjacent to the 5S rDNA cluster. Additional 5S rDNA loci were present on the autosome pair 8, X₁ chromosome, and in the presumed fusion point on the Y chromosome. The probe for telomeric repeat motif (TTAGGG)_n revealed signals of variable intensities at the ends of all chromosomes except for the Y chromosome, where no detectable signals were evidenced. Male-to-female comparative genomic hybridization revealed no sex-specific or sex-biased repetitive DNA accumulations, suggesting a presumably low level of neo-Y chromosome differentiation. We provide evidence that rDNA sites might have played a role in the formation of this putative multiple sex chromosome system and that chromosome fusions originate through different mechanisms among different Rineloricaria species.     

Gene mapping studies confirm the homology between the platypus X and echidna X1 chromosomes and identify a conserved ancestral monotreme X chromosome

The identification of the sex chromosomes in the three extant species of Prototherian mammals (the monotremes) is complicated by their involvement in a multivalent translocation chain at the first division of male meiosis. The platypus X chromosome, identified by the presence of two copies in females and one in males, has been found to possess a suite of genes that have been mapped to the X chromosomes of all eutherian and metatherian mammals. We have extended gene mapping studies to a member of the only other extant monotreme family, the echidna, which has a G-band equivalent X₁ chromosome, as well as a smaller X₂. We find that the five human X-linked genes (G6PD, GDX, F9, AR and MCF2) map to the echidna X₁ chromosome in locations equivalent to those on the platypus X. These results confirm that the echidna X₁ is the original X chromosome in this species, and identify a conserved ancestral monotreme X chromosome.     

New insights into sex chromosome evolution in anole lizards (Reptilia,Dactyloidae)

Anoles are a clade of iguanian lizards that underwent an extensive radiation between 125 and 65 million years ago. Their karyotypes show wide variation in diploid number spanning from 26 (Anolis evermanni) to 44 (A. insolitus). This chromosomal variation involves their sex chromosomes, ranging from simple systems (XX/XY), with heterochromosomes represented by either micro- or macrochromosomes, to multiple systems (X₁X₁X₂X₂/X₁X₂Y). Here, for the first time, the homology relationships of sex chromosomes have been investigated in nine anole lizards at the whole chromosome level. Cross-species chromosome painting using sex chromosome paints from A. carolinensis, Ctenonotus pogus and Norops sagrei and gene mapping of X-linked genes demonstrated that the anole ancestral sex chromosome system constituted by microchromosomes is retained in all the species with the ancestral karyotype (2n?=?36, 12 macro- and 24 microchromosomes). On the contrary, species with a derived karyotype, namely those belonging to genera Ctenonotus and Norops, show a series of rearrangements (fusions/fissions) involving autosomes/microchromosomes that led to the formation of their current sex chromosome systems. These results demonstrate that different autosomes were involved in translocations with sex chromosomes in closely related lineages of anole lizards and that several sequential microautosome/sex chromosome fusions lead to a remarkable increase in size of Norops sagrei sex chromosomes.     

Morphologically differentiated sex chromosomes in neotropical freshwater fish

A general survey of the occurrence of morphologically differentiated sex chromosomes in the neotropical freshwater fishes is presented. The total number of 32 occurrences involving simple XX-XY and ZZ-ZW, and multiple X₁X₂Y, XY₁Y₂ and ZW₁W₂ sex chromosome systems is described, with comments on the aspects of sex chromosome evolution in this fish fauna. The occurrence of different sex chromosome systems in related species of the same genus, or in different populations of the same nominal species, involving male and sometimes female heterogamety, and differences in the molecular composition of sex-linked heterochromatin, are considered as indicative of the early stage of sex chromosomes evolution in fish.     

Stability of a multiple X chromosome system and associated B chromosomes in birch aphids (Euceraphis spp.; Homoptera: Aphididae)

Autosomal dissociations are a common feature of aphid karyotype evolution, but multiple X chromosome systems are rare. Birch-feeding aphids of the genus Euceraphis, however, have X₁X₂O males as a general rule, X₁ being always much larger than X₂. Only one species has XO males, and this condition appears to be secondary. Most Euceraphis karyotypes also have one or more, usually heterochromatic, elements that occur in the same numbers in both males and females, yet behave like X chromosomes at male and female meiosis I. They appear to be supernumerary, non-functional X chromosomes, although showing greater within-species stability in size and number than typical B chromosomes. Euceraphis gillettei forms a separate group within the genus and feeds on alders (Alnus species), yet has a similar system, and the two most closely related genera, Symydobius and Clethrobius, also have additional chromosomal elements possibly representing non-functional X chromosomes. Thus the multiple X chromosome system in these aphids seems to be a primitive condition.     

Turnover of sex chromosomes and speciation in fishes

Closely related species of fishes often have different sex chromosome systems. Such rapid turnover of sex chromosomes can occur by several mechanisms, including fusions between an existing sex chromosome and an autosome. These fusions can result in a multiple sex chromosome system, where a species has both an ancestral and a neo-sex chromosome. Although this type of multiple sex chromosome system has been found in many fishes, little is known about the mechanisms that select for the formation of neo-sex chromosomes, or the role of neo-sex chromosomes in phenotypic evolution and speciation. The identification of closely related, sympatric species pairs in which one species has a multiple sex chromosome system and the other has a simple sex chromosome system provides an opportunity to study sex chromosome turnover. Recently, we found that a population of threespine stickleback (Gasterosteus aculeatus) from Japan has an X₁X₂Y multiple sex chromosome system resulting from a fusion between the ancestral Y chromosome and an autosome, while a sympatric threespine stickleback population has a simple XY sex chromosome system. Furthermore, we demonstrated that the neo-X chromosome (X ₂) plays an important role in phenotypic divergence and reproductive isolation between these sympatric stickleback species pairs. Here, we review multiple sex chromosome systems in fishes, as well as recent advances in our understanding of the evolutionary role of sex chromosome turnover in stickleback speciation.     

Whole chromosome painting reveals independent origin of sex chromosomes in closely related forms of a fish species

The wolf fish Hoplias malabaricus includes well differentiated sex systems (XY and X₁X₂Y in karyomorphs B and D, respectively), a nascent XY pair (karyomorph C) and not recognized sex chromosomes (karyomorph A). We performed the evolutionary analysis of these sex chromosomes, using two X chromosome-specific probes derived by microdissection from the XY and X₁X₂Y sex systems. A putative-sex pair in karyomorph A was identified, from which the differentiated XY system was evolved, as well as the clearly evolutionary relationship between the nascent XY system and the origin of the multiple X₁X₂Y chromosomes. The lack of recognizable signals on the sex chromosomes after the reciprocal cross-FISH experiments highlighted that they evolved independently from non-homologous autosomal pairs. It is noteworthy that these distinct pathways occur inside the same nominal species, thus exposing the high plasticity of sex chromosome evolution in lower vertebrates. Possible mechanisms underlying this sex determination liability are also discussed.     

Fluorescence in situ hybridization establishes homology between human and silvered leaf monkey chromosomes,reveals reciprocal translocations between chromosomes homologous to human Y/5, 1/9, and 6/16, and delineates an X1X2Y1Y2/X1X1X2X2 sex-chromosome system

We employed in situ hybridization of chromosome-specific DNA probes (“chromosome painting”) of all human chromosomes to establish homologies between the human and the silvered lead monkey karyotypes (Presbytis cristata 2n=44). The 24 human paints gave 30 signals on the haploid female chromosome set and 34 signals on the haploid male chromosome set. This difference is due to a reciprocal translocation between the Y and an autosome homologous to human chromosome 5. This Y/autosome reciprocal translocation which is unique among catarrhine primates has produced a X₁X₂Y₁Y₂/X₁X₁X₂X₂ sex-chromosome system. Although most human syntenic groups have been maintained in the silvered leaf monkey chromosomes homologous to human chromosomes 14 and 15, 21 and 22 have experienced Robertsonian fusions. Further, the multiple FISH signals provided by libraries to human chromosomes 1/9, 6/16 indicate that these chromosomes have been split by reciprocal translocations. G-banding analysis shows three different forms of chromosome 1 (X₂) which differ by a complex series of inversions in the 10 individuals karyotyped. Comparisons with the hybridization patterns in hylobatids (gibbons and siamang) demonstrate that resemblances in chromosomal morphology and banding previously taken to indicate a special phylogenetic relationship between gibbons and colobines are due to convergence. A. J. Phys. Anthropol. 102:315–327, 1997. © 1997 Wiley-Liss, Inc.     

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 CONTRIBUTION OF FEMALE MEIOTIC DRIVE TO THE EVOLUTION OF NEO‐SEX CHROMOSOMES

Sex chromosomes undergo rapid turnover in certain taxonomic groups. One of the mechanisms of sex chromosome turnover involves fusions between sex chromosomes and autosomes. Sexual antagonism, heterozygote advantage, and genetic drift have been proposed as the drivers for the fixation of this evolutionary event. However, all empirical patterns of the prevalence of multiple sex chromosome systems across different taxa cannot be simply explained by these three mechanisms. In this study, we propose that female meiotic drive may contribute to the evolution of neo‐sex chromosomes. The results of this study showed that in mammals, the XY₁Y₂ sex chromosome system is more prevalent in species with karyotypes of more biarmed chromosomes, whereas the X₁X₂Y sex chromosome system is more prevalent in species with predominantly acrocentric chromosomes. In species where biarmed chromosomes are favored by female meiotic drive, X‐autosome fusions (XY₁Y₂ sex chromosome system) will be also favored by female meiotic drive. In contrast, in species with more acrocentric chromosomes, Y‐autosome fusions (X₁X₂Y sex chromosome system) will be favored just because of the biased mutation rate toward chromosomal fusions. Further consideration should be given to female meiotic drive as a mechanism in the fixation of neo‐sex chromosomes.     

Pairing and segregation of the sex chromosomes in X1X2-males of Dysdercus intermedius with a note on the kinetic organization of Heteropteran chromosomes

The existence of an X₁X₂-mode of sex determination is confirmed by a study of all meiotic stages in the male cotton stainer (X₁X₂ and pertinent stages in the female (X₁X₁ X₂X₂). In the male, the X-chromosomes are heterochromatic and pair end-to-end in early meiotic prophase. At diakinesis, they disjoin and align side-by-side in the center of the spindle, forming a pseudotetrad. Anaphase I is equational for the sex chromosomes. At late anaphase or telophase, X₁ and X₂ join end-to-end but form spindle fiber connections to only one of the poles of the metaphase II spindle, leading to one daughter cell without X chromosomes and one with both X₁ and X₂. An attempt is made to explain sex chromosome pairing and orientation on the basis of a telocentric organization of meiotic chromosomes. The apparent differences in the kinetic organization of mitotic and meiotic chromosomes in Heteroptera are discussed.     

Differentiation and evolutionary relationships in Erythrinus erythrinus (Characiformes, Erythrinidae): comparative chromosome mapping of repetitive sequences

Erythrinus erythrinus presents extensive karyotypic diversity, with four karyomorphs (A–D) differing in the number of chromosomes, karyotype structure or sex chromosomes systems. Karyomorph A has 2n = 54 chromosomes in males and females without heteromorphic sex chromosomes, while karyomorph C has 2n = 52 chromosomes in females and 2n = 51 chromosomes in males, due a X₁X₁X₂X₂/X₁X₂Y sex chromosome system. Three allopatric populations of the karyomorph A and one population of the karyomorph C were now in deep investigated by molecular cytogenetic analyses, using repetitive DNAs as probes. The results reinforced the relatedness among populations of the karyomorph A, despite their large geographic distribution. Karyomorph C, however, showed a remarkably difference in the genomic constitution, especially concerning the amount and distribution of the 5S rDNA and Rex3 sequences on chromosomes. In addition, although karyomorphs C and D share several features, exclusive chromosomal markers show the derivative evolutionary pathway between them. Thus, besides the classical chromosomal rearrangements, the repetitive DNAs were useful tools to reveal the biodiversity, relatedness and differentiation of this fish group. The chromosomal set strongly corroborates that E. erythrinus corresponds to a species complex instead of a single biological entity.     

Fissions,fusions, and translocations shaped the karyotype and multiple sex chromosome constitution of the northeast‐Asian wood white butterfly,Leptidea amurensis

Previous studies have shown a dynamic karyotype evolution and the presence of complex sex chromosome systems in three cryptic Leptidea species from the Western Palearctic. To further explore the chromosomal particularities of Leptidea butterflies, we examined the karyotype of an Eastern Palearctic species, Leptidea amurensis. We found a high number of chromosomes that differed between the sexes and slightly varied in females (i.e. 2n = 118–119 in females and 2n = 122 in males). The analysis of female meiotic chromosomes revealed multiple sex chromosomes with three W and six Z chromosomes. The curious sex chromosome constitution [i.e. W_1–3/Z_1–6 (females) and Z_1–6/Z_1–6 (males)] and the observed heterozygotes for a chromosomal fusion are together responsible for the sex‐specific and intraspecific variability in chromosome numbers. However, in contrast to the Western Palearctic Leptidea species, the single chromosomal fusion and static distribution of cytogenetic markers (18S rDNA and H3 histone genes) suggest that the karyotype of L. amurensis is stable. The data obtained for four Leptidea species suggest that the multiple sex chromosome system, although different among species, is a common feature of the genus Leptidea. Furthermore, inter‐ and intraspecific variations in chromosome numbers and the complex meiotic pairing of these multiple sex chromosomes indicate the role of chromosomal fissions, fusions, and translocations in the karyotype evolution of Leptidea butterflies.     

Chromosome banding in amphibia

A cytogenetic study performed on a population of the South American leptodactylid frog Eleutherodactylus maussi revealed multiple sex chromosomes of the X₁X₁X₂X₂/X₁X₂Y (=XXAA/XXA^Y) type. The diploid chromosome number is 2n=36 in all females and 2n=35 in most males. The multiple sex chromosomes originated by a centric fusion between the original Y chromosome and a large autosome. In male meiosis the X₁X₂Y (=XXA^Y) multiple sex chromosomes form a classical trivalent configuration. E. maussi is the first species discovered in the class Amphibia that is distinguished by a system of multiple sex chromosomes. Only one single male was found in the population with 2n=36 chromosomes and lacking the Y-autosomal fusion. This karyotype (XYAA) is interpreted as the ancestral condition, preceding the occurrence of the Y-autosome fusion.by H.C. Macgregor