# Comparative Chromosome Painting Clarifies the Intraspecific Chromosomal Variation in Two Ctenomys Species (Rodentia: Ctenomyidae)

## TL;DR

This study uses chromosome painting to uncover chromosomal rearrangements in two Ctenomys rodent species, revealing insights into their evolutionary divergence.

## Contribution

The study identifies shared chromosomal rearrangements within and between Ctenomys species using comparative chromosome painting.

## Key findings

- Chromosomal rearrangements were found to differentiate cytotypes within and between C. minutus and C. lami.
- Some chromosomes and chromosomal segments remain conserved across C. minutus, C. lami, and C. flamarioni.
- The findings highlight the role of chromosomal rearrangements in the diversification of the Ctenomys genus.

## Abstract

Ctenomys minutus and Ctenomys lami are rodents known for their karyotypic diversity. The C. minutus diploid number ranges from 2n = 42–50, divided into seven parental cytotypes. While C. lami diploid number ranges from 2n = 50–58, it is divided into four parental cytotypes. To understand the chromosomal evolution of these species, we used whole-chromosome painting with chromosomic probes from Ctenomys flamarioni. We identified chromosomal rearrangements shared among C. minutus and C. lami, and within different cytotypes from within each species. Moreover, several conserved chromosomes across all three Ctenomys species were observed. These rearrangements shed light on the chromosomal evolution of the genus Ctenomys, particularly within the torquatus group, suggesting that these rearrangements have a role on diversification.

Background: Ctenomys is a subterranean rodent genus known for exhibiting the highest levels of chromosome variation, both among species (2n = 10 to 70) and within species. Ctenomys minutus is particularly notable for its extensive chromosomal diversity, comprising the greatest number of described cytotypes within this genus. In contrast, Ctenomys lami presents the highest degree of karyotypic variation within a comparatively restricted geographic range. Both species inhabit the coastal plain of southern Brazil: C. minutus occurs in dunes and sandy fields extending from Laguna (Santa Catarina State) to São José do Norte (Rio Grande do Sul State), whereas C. lami is restricted to the “Coxilha das Lombas” region, which lies parallel to the distribution of C. minutus in Rio Grande do Sul State. Despite their close evolutionary relationship and the absence of external morphological differences, the mechanism underlying their karyotypic divergence remains poorly understood. Methods: In this study, we applied whole-chromosome painting using probes from Ctenomys flamarioni to investigate chromosomal evolution in C. minutus and C. lami. Results: The resulting homology maps revealed a variety of chromosomal rearrangements that differentiate cytotypes both within and between these species. Comparative analyses demonstrated substantial karyotypic divergence from C. flamarioni, although some entire chromosomes and large chromosomal segments remained conserved between C. minutus and C. lami. Our findings underscore the critical role of chromosomal rearrangements in shaping the diversification of Ctenomys. Additionally, we identified shared chromosomal rearrangements in C. minutus and C. lami, which are likely restricted to the torquatus group. Conclusions: These rearrangements provide new insights into the processes driving chromosomal evolution in genus Ctenomys.