Chromosomal evolution of tribe Oryzomyini (Rodentia: Cricetidae: Sigmodontinae)

Karyotypic Reshuffling in the Genus Rhipidomys (Rodentia: Cricetidae: Sigmodontinae) Revealed by Zoo-FISH

INTRODUCTION

Rhipidomys is the second most specious and the most widespread genus of the tribe Thomasomyini. Chromosomal data have been an important tool in the taxonomy of the group that presents low variability of diploid number (2n) and highly variable fundamental numbers (FNs). Despite such diversity, the genus has been studied mainly by classical and banding cytogenetic techniques.

METHODS

This study performed a comparative study between R. emiliae (2n = 44, FN = 52), R. macrurus (2n = 44, FN = 49), R. nitela (2n = 50, FN = 71), and R. mastacalis (2n = 44, FN = 72) using chromosome painting probes of two Oryzomyini species.

RESULTS

Our analysis revealed pericentric inversion as the main rearrangement involved in the karyotype evolution of the group, although tandem fusions/fissions were also detected. In addition, we detected eight syntenic associations exclusive of the genus Rhipidomys, and three syntenic associations shared between species of the tribe Thomasomyini and Oryzomyini.

CONCLUSION

Comparative cytogenetic analysis by ZOO-FISH on genus Rhipidomys supports a pattern of chromosomal rearrangement already suggested by comparative G-banding. However, the results suggest that karyotype variability in the genus could also involve the occurrence of an evolutionary new centromere.

Chromosomal Radiation: A model to explain karyotypic diversity in cryptic species

We present a concept that explains the pattern of occurrence of widely distributed organisms with large chromosomal diversity, large or small molecular divergence, and the insufficiency or absence of morphological identity. Our model is based on cytogenetic studies associated with molecular and biological data and can be applied to any lineage of sister species, chronospecies, or cryptic species. Through the evaluation of the karyotypic macrostructure, as the physical location of genes e satellites DNAs, in addition to phylogenetic reconstructions from mitochondrial and nuclear genes, per example, we have observed morphologically indistinguishable individuals presenting different locally fixed karyomorphs with phylogeographic discontinuity. The biological process behind this pattern is seen in many groups of cryptic species, in which variation lies mainly in the organization of their genomes but not necessarily in the ecosystems they inhabit or in their external morphology. It's similar to the processes behind other events observed in the distribution of lineages. In this work, we explore the hypothesis of a process analogous to ecological-evolutionary radiation, which we called Chromosomal Radiation. Chromosomal Radiation can be adaptive or non-adaptive and applied to different groups of organisms.

Characterization of repetitive DNA on the genome of the marsh rat Holochilus nanus (Cricetidae: Sigmodontinae)

Repetitive DNA are sequences repeated hundreds or thousands of times and an abundant part of eukaryotic genomes. SatDNA represents the majority of the repetitive sequences, followed by transposable elements. The species Holochilus nanus (HNA) belongs to the rodent tribe Oryzomyini, the most taxonomically diverse of Sigmodontinae subfamily. Cytogenetic studies on Oryzomyini reflect such diversity by revealing an exceptional range of karyotype variability. However, little is known about the repetitive DNA content and its involvement in chromosomal diversification of these species. In the search for a more detailed understanding about the composition of repetitive DNA on the genome of HNA and other species of Oryzomyini, we employed a combination of bioinformatic, cytogenetic and molecular techniques to characterize the repetitive DNA content of these species. RepeatExplorer analysis showed that almost half of repetitive content of HNA genome are composed by Long Terminal Repeats and a less significant portion are composed by Short Interspersed Nuclear Elements and Long Interspersed Nuclear Elements. RepeatMasker showed that more than 30% of HNA genome are composed by repetitive sequences, with two main waves of repetitive element insertion. It was also possible to identify a satellite DNA sequence present in the centromeric region of Oryzomyini species, and a repetitive sequence enriched on the long arm of HNA X chromosome. Also, comparative analysis between HNA genome with and without B chromosome did not evidence any repeat element enriched on the supernumerary, suggesting that B chromosome of HNA is composed by a fraction of repeats from all the genome.

Geographic Mosaic of Extensive Genetic Variations in Subterranean Mole Voles Ellobius alaicus as a Consequence of Habitat Fragmentation and Hybridization

Restricted mobility, sociality, and high inbreeding—characteristic for subterranean mammals—lead to rapid changes in their genome structure. Up to now, the Alay mole vole Ellobius alaicus was a data-deficient species; its spatial and phylogenetic relationships with a sibling species, E. tancrei, were not clarified. We carried out a genetic analysis including differential G-banding of chromosomes and mitochondrial (cytb) and nuclear gene (XIST and IRBP) sequencing. The phylogenetic reconstruction based on cytb represented the expected phylogenetic relationships of two species. Using the XIST, we revealed two new lineages among E. alaicus from the Alay Valley (Southern Kyrgyzstan). Analysis of IRBP demonstrated presence of the specific genotype in most of E. alaicus specimens, but also revealed the haplotype, typical for E. tancrei, in some Alay mole voles. The results may be explained as persistence of ancestral gene polymorphism in E. alaicus or limited interspecific hybridization with E. tancrei. Several chromosomal forms were revealed in E. alaicus in the Alay Valley. We propose that ‘mosaic’ genetic polymorphism might appear in E. alaicus due to fragmentation of their habitats in highlands of the Alay Valley, Tien Shan, and Pamir-Alay as well as due to hybridization with E. tancrei or persistence of ancestral polymorphisms.