Sex chromosomes pairing in two Arvicolidae species: Microtus nivalis and Arvicola sapidus

Abstract

Arvicolid rodents present both synaptic and asynaptic sex chromosomes. We analyzed the pairing behaviour of sex chromosomes in two species belonging to this rodent group (Microtus nivalis and Arvicola sapidus). At pachynema, the sex chromosomes of both species paired in a small region while the rest remain unsynapsed. Consequently at metaphase I, sex chromosomes present end-to-end association. Thus, the pairing behaviour of sex chromosomes in these species is very similar to that previously described for other arvicolid rodents and for most mammals. According to this, we propose that synaptic sex chromosomes were the ancestral condition in the family Arvicolidae, including the genus Microtus. The phylogenetic origin of the asynaptic sex chromosomes in the genus Microtus would have arisen once in the lineage that originated the species M. arvalis/agrestis and related species, while the lineage that originated the species M. oeconomous and related species conserved synaptic chromosomes. Furthermore, the phylogenetic relationships between the genus Microtus, Chionomys and Pitymys are discussed in relation to the synaptic behaviour of sex chromosomes.

Changes in the numbers of chromosomes and sex determination system in bushcrickets of the genus Odontura (Orthoptera: Tettigoniidae: Phaneropterinae)

“…The pairing between sex chromosomes is restricted to very short terminally located heterochromatic segments of the neo-X and neo-Y. Similar meiotic behaviour of sex chromosomes is reported in species of Arvicolidae (Megías-Nogales et al, 2003). These mechanisms result in the appearence of loop-like or "parachute"-like associations of sex chromosomes, typical of Coleoptera (e.g.…”

A repeat DNA sequence from the Y chromosome in species of the genus Microtus

“…Furthermore, non-recombining status is extended to the whole Y chromosome in some of the species analysed (M. cabrerae, M. arvalis, M. agrestis, M. guentheri), since they present asynaptic sex chromosomes (Megı´as-Nogales et al 2003). In fact, this ampli¢cation mechanism is not valid for the heterochromatic blocks of both X and Y chromosomes in Microtus because these regions never pair at meiosis, whatever the condition, synaptic or asynaptic, of the sex chromosomes (Singh et al 2000, Meg as-Nogales et al 2003.…”

Sex chromosomes, sex determination, and sex-linked sequences in Microtidae

“…One of them stated that large heterochromatic regions on sex chromosomes could interfere with synapsis (Solari and Ashley, 1977). However, asynapsis also occurs in species without heterochromatic regions on sex chromosomes (Ashley et al, 1989;Carnero et al, 1991;Borodin et al, 1995;Megías-Nogales et al, 2003). Also, the loss or malfunction of the pairing regions in sex chromosomes has been proposed as the cause of asynapsis Jiménez et al, 1991).…”

“…The alternative to this hypothesis is the existence of a common origin for the species with asynaptic sex chromosomes. Megías-Nogales et al (2003) have suggested that asynaptic sex chromosomes originated only once in the ancestral species of the lineage that gave rise to the M. arvalis/agrestis group of species, while in the rest of Microtus species the ancestral synaptic condition remained.…”

“…The "synaptic group" includes species with pairing and synapsis between sex chromosomes at pachynema. On the other hand, the "asynaptic group" includes those species with sex chromosomes that do not pair or synapse at pachynema (see Megías-Nogales et al, 2003).…”