The emergence of a new sex-system (XX/XY1Y2) suggests a species complex in the “monotypic” rodent Oecomys auyantepui (Rodentia, Sigmodontinae) (original) (raw)
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International Journal of Molecular Sciences
Comparative chromosome-painting analysis among highly rearranged karyotypes of Sigmodontinae rodents (Rodentia, Cricetidae) detects conserved syntenic blocks, which are proposed as chromosomal signatures and can be used as phylogenetic markers. In the Akodontini tribe, the molecular topology (Cytb and/or IRBP) shows five low-supported clades (divisions: “Akodon”, “Bibimys”, “Blarinomys”, “Oxymycterus”, and “Scapteromys”) within two high-supported major clades (clade A: “Akodon”, “Bibimys”, and “Oxymycterus”; clade B: “Blarinomys” and “Scapteromys”). Here, we examine the chromosomal signatures of the Akodontini tribe by using Hylaeamys megacephalus (HME) probes to study the karyotypes of Oxymycterus amazonicus (2n = 54, FN = 64) and Blarinomys breviceps (2n = 28, FN = 50), and compare these data with those from other taxa investigated using the same set of probes. We strategically employ the chromosomal signatures to elucidate phylogenetic relationships among the Akodontini. When we ...
Chromosoma, 2008
We describe the outcome of a comprehensive cytogenetic survey of the common mole-rat, Cryptomys hottentotus, based on G and C banding, fluorescence in situ hybridisation and the analysis of meiotic chromosomes using immunostaining of proteins involved in the formation of synaptonemal complex (SCP1 and SCP3). We identified the presence of a Y-autosome translocation that is responsible for a fixed diploid number difference between males (2n=53) and females (2n=54), a character that likely defines the C. hottentotus lineage. Immunostaining, combined with C banding of spermatocytes, revealed a linearised sex trivalent with X 1 at one end and X 2 at the other, with evidence of reduced recombination between Y and X 2 that seems to be heterochromatin dependant in the C. hottentotus lineage. We suggest that this could depict the likely initial step in the differentiation of a true neo-X, and that this may mimic an early stage in the mammalian meiotic chain formation, an evolutionary process that has been taken to an extreme in a monotreme mammal, the platypus.
2008
The Bathyergidae are subterranean rodents endemic to Africa south of the Sahara. They are characterised by divergent diploid numbers that range from 2n=40 in Fukomys mechowi to 2n=78 in F. damarensis. In spite of this variation there is limited understanding of the events that shaped the extant karyotypes and in an attempt to address this, and to shed light on the mode and tempo of chromosomal evolution in the African mole-rats, a detailed analysis of both the autosomal and sex chromosome components of the genome was undertaken. In addition to G-and Cbanding, Heterocephalus glaber (2n=60) flow-sorted painting probes were used to conduct cross-species chromosome painting among bathyergids. This allowed the detection of a balanced sex chromosome-autosome translocation in F. mechowi that involved a complex series of rearrangements requiring fractionation of four H. glaber autosomes and the subsequent translocation of segments to sex chromosomes and to the autosomal partners. The fixation of this rare rearrangement has probably been favoured by the presence of an intercalary heterochromatic block (IHB) that was detected at the boundary with the translocated autosomal segment. Male meiosis in Cryptomys, the Fukomys sister clade, was investigated by immunostaining of the SCP1 and SCP3 proteins involved in the formation of the synaptonemal complex. This allowed confirmation of a Y-autosome translocation that is shared by C. hottentotus subspecies. We discuss reduced recombination between Y and X 2 that seems to be heterochromatin dependent in the C hottentotus lineage, and the implications this holds for the evolution of a meiotic sex chromosome chain such as has been observed in platypus. By extending cross-species chromosome painting to Bathyergus janetta, F. damarensis, F. darlingi and Heliophobius argenteocinereus, homologous chromosomal regions across a total of 11 species/subspecies and an outgroup were examined using cladistic and bioinformatics approaches. The results show that Bathyergus, Georychus and Cryptomys are karyotypically highly conserved in comparison to Heterocephalus, Heliophobius and Fukomys. Fukomys in particular is characterised by a large number of rearrangements that contrast sharply with the conservative Cryptomys. The occurrence and fixation of rearrangements in these species has probably been facilitated by vicariance in combination with life history traits that are particular to these mammals.
Scientific Reports, 2019
Multiple sex chromosome systems have been described for several mammalian orders, with different species from the same genus sharing the same system (e.g., X 1 X 2 Y or XY 1 Y 2). This is important because the translocated autosome may be influenced by the evolution of the recipient sex chromosome, and this may be related to speciation. It is often thought that the translocation of an autosome to a sex chromosome may share a common origin among phylogenetically related species. However, the neo-X chromosomes of Proechimys goeldii (2n = 24 , 25 /NFa = 42) and Proechimys gr. goeldii (2n = 16 , 17 / NFa = 14) have distinct sizes and morphologies that have made it difficult to determine whether they have the same or different origins. This study investigates the origins of the XY 1 Y 2 sex chromosome determination system in P. goeldii (PGO) and P. gr. goeldii (PGG) and elucidates the chromosomal rearrangements in this low-diploid-number group of Proechimys species. Toward this end, we produced whole-chromosome probes for P. roberti (PRO; 2n = 30 /NFa = 54) and P. goeldii (2n = 25 /NFa = 42) and used them in comparative chromosomal mapping. Our analysis reveals that multiple translocations and inversions are responsible for the karyotype diversity of these species, with only three wholechromosomes conserved between PRO and PGO and eight between PGO and PGG. Our data indicate that multiple sex chromosome systems have originated twice in Proechimys. As small populations are prone to the fixation of chromosomal rearrangements, we speculate that biological features of Rodentia contribute to this fixation. We also highlight the potential of these rodents as a model for studying sex chromosome evolution. Reproductive isolation is an important step in the speciation process. Speciation mediated by geographic isolation has been well documented and is generally accepted 1-4 , but the differentiation process between spatially contiguous populations is complex and not well documented. One particular type of sympatric speciation-that mediated by chromosomal changes-has been observed in both plants 5 and animals 6. Chromosomal rearrangements have long been discussed for their ability to reduce the fertility of heterozygous individuals 7. They can also reduce gene flow by suppressing recombination between the rearranged and parental segments, extending the effects of gene isolation 6. An example of the role of chromosomal rearrangements in the process of lineage diversification was found among two populations of Gasterosteus aculeatus that show distinct chromosomal sex determination systems (XX/XY and X 1 X 2 Y/X 1 X 1 X 2 X 2) 8. The authors showed that males with the X 1 X 2 Y system presented different spine sizes and courtship behavior compared to males with the XY system, and that these differentiated characteristics were associated with the neo-X chromosome. The phenotypes present in X 1 X 2 Y individuals may have arisen after the origination of the neo-Y and accumulated on the neo-X chromosome during 1.5-2 Ma (million years ago), which points to reproductive isolation mediated by an autosomal-sexual chromosomal translocation between closely related species 8 .
Cytogenetic and Genome Research, 2011
Chromosome painting with Akodon paranaensis chromosome 21 (a small metacentric akodont marker) paint revealed total homology with the smallest acrocentric Deltamys sp. chromosome, DSP19. This suggests the occurrence of a pericentric inversion or centromeric shift when compared to other akodontines, with a posterior tandem rearrangement giving rise to DKE4. In DKE, large blocks of pericentromeric constitutive heterochromatin are present on the autosomes and the X, and the Y/autosome has an entirely heterochromatic short arm. In DSP, small heterochromatic blocks are observed on autosomes and X, and the Y is a very small, mostly heterochromatic acrocentric. The cytogenetic analyses suggest that the Deltamys sp. karyotype is ancestral, with the derived condition resulting from a tandem fusion (DSP7 + DSP19) and the Y/autosome translocation giving rise to the multiple sex chromosome system. The autosomal rearrangements, the differences in CBG-banding patterns and Ag-NOR localization, as well as the presence of X 1 X 1 X 2 X 2 /X 1 X 2 Y and XX/XY sex determination mechanisms, possibly acting as a reproductive barrier, and the phylogenetic position within the Deltamys genus, with high genetic divergence, call for a taxonomic review of the genus.
PloS one, 2015
Sigmodontinae rodents represent one of the most diverse and complex components of the mammalian fauna of South America. Among them most species belongs to Oryzomyini and Akodontini tribes. The highly specific diversification observed in both tribes is characterized by diploid complements, which vary from 2n = 10 to 86. Given this diversity, a consistent hypothesis about the origin and evolution of chromosomes depends on the correct establishment of synteny analyzed in a suitable phylogenetic framework. The chromosome painting technique has been particularly useful for identifying chromosomal synteny. In order to extend our knowledge of the homeological relationships between Akodontini and Oryzomyini species, we analyzed the species Akodon montensis (2n = 24) and Thaptomys nigrita (2n = 52) both from the tribe Akodontini, with chromosome probes of Hylaeamys megacephalus (2n = 54) of the tribe Oryzomyini. The results indicate that at least 12 of the 26 autosomes of H. megacephalus sho...
The complement of Akodon azarae comprises 38 chromosomes. With the ex ception of pair 18 which is metacentric, all the other autosomes are acrocentric. X-chromosomes are the second in size (7.89% of the haploid female set) and have a subterminal centromere (arm index 7.9); accordingly, the X-chromosomes can be easily recognized. The Y chromosome in some cases can also be identified with accuracy; it is one of the smallest chromosomes in the set (1.15% of the haploid male set), it has a subterminal centromere (although occasionally it may appear as acrocentric) and usually it exhibits a slightly positive heteropycnosis (Bianchi and Contreras 1967, Bianchi et al. 1968). Akodon azarae has a sex chromosome polymorphism which gives rise to five different varieties of females: a) females with a normal XX sex pair, b) females with a deletion of the short arm of one X chromosome (Xx(s)), c) females with an extensive deletion of the long arm of one X chromosome (Xx(l)), d) XO females, and e) XO/Xx(l)) mosaic specimens. In the case of partially deleted females the chromosomes x(s) and x(l) are always inactive and involved in the mechanism of sex dosage compensation. Males always have a normal XY sex pair and no case of OY,x(s) Y or x(l) Y has ever been found (Bianchi et al. 1967, 1968); these zygotes are, therefore, assumed to be non-viable. Two hypotheses have been proposed to explain the findings in Akodon azarae. In the first it is assumed that XX, Xx and XO females are fertile and able to produce X, x and 0 ova to give XX, Xx and XO females after fertilization by X-bearing sperm. The second hypothesis proposes and extreme mechanism of sex dosage compensation ranging from X-chromosome inactivation to X chromosome elimi nation. Thus, X chromosome inactivation would give rise to sex chromatin positive XX females; complete X chromosome elimination would produce XO fe males; deletion of part of the inactivated X-chromosome would give rise to Xx(s) or Xx(l) specimens; and finally the combination of the latter two possibilities would be responsible for the Xx(l)/XO mosaics (Bianchi and Contreras 1967, Bianchi 1973). The existence of XX and XY females in natural populations of the wood lemming Myopus schisticolor has been reported by Fregda et al. (1976). The differentiation
Chromosome Research, 2009
Traditionally comparative cytogenetic studies are based mainly on banding patterns. Nevertheless, when dealing with species with highly rearranged genomes, as in Akodon species, or with other highly divergent species, cytogenetic comparisons of banding patterns prove inadequate. Hence, comparative chromosome painting has become the method of choice for genome comparisons at the cytogenetic level since it allows complete chromosome probes of a species to be hybridized in situ onto chromosomes of other species, detecting homologous genomic regions between them. In the present study, we have explored the highly rearranged complements of the Akodon species using reciprocal chromosome painting through speciesspecific chromosome probes obtained by chromosome sorting. The results revealed complete homology among the complements of Akodon sp. n. (ASP), 2n=10; Akodon cursor (ACU), 2n=15; Akodon montensis (AMO), 2n=24; and Akodon paranaensis (APA), 2n=44, and extensive chromosome rearrangements have been detected within the species with high precision. Robertsonian and tandem rearrangements, pericentric inversions and/or centromere repositioning, paracentric inversion, translocations, insertions, and breakpoints, where chromosomal rearrangements, seen to be favorable, were observed. Chromosome painting using the APA set of 21 autosomes plus X and Y revealed eight syntenic segments that are shared with A. montensis, A. cursor, and ASP, and one syntenic segment shared by A. montensis and A. cursor plus five exclusive chromosome associations for A. cursor and six for ASP chromosome X, except for the heterochromatin region of ASP X, and even chromosome Y shared complete homology among the species. These data indicate that all those closely related species have experienced a recent extensive process of autosomal rearrangement in which, except for ASP, there is still complete conservation of sex chromosomes homologies.
Mammalian Biology, 2009
The populations of several species of Akodon present, besides XX females, a variable proportion of fertile XY females. In Akodon azarae, a correspondence exists between the X-chromosome C-banding pattern and the sexual phenotype of XY individuals: males carry a determinate X-chromosome type, defined by its C-banding pattern, and XY females, any of two others. To confirm the relation between X-chromosome type and the XY female condition and to investigate the hereditary transmission of these different X-chromosomes, we analyzed 50 animals captured in the field and 95 individuals corresponding to the F 1 and F 2 offspring of 16 crosses. It was seen that the correlation between X type and the sexual phenotype of XY animals is retained, and that the three X types are transmitted to the progeny. It was also observed that the male offspring of XY females receive the X-chromosome from their male parents and the Y from their mothers. These results strongly support the causal role of an X-borne mutation in A. azarae XY sex reversal, and discard a mutation of the Y-chromosome as the sole basis of this phenomenon.