Microtus cabrerae

Abstract: In mammals, sex determination is controlled by the Y-linked gene SRY. Although SRY is male-specific in most eutherian and marsupial species, with a single copy on the Y chromosome, several rodent species have multiple Y-linked copies of SRY, and two mole-vole species of the genus Ellobius determine sex without the Y chromosome or the SRY gene. We searched for homologs of SRY in three vole species of the genus Microtus and concluded that this gene is not male-specific in M. cabrerae, as it is present in multiple, polymorphic copies in both males and females. In contrast, SRY is male-specific in the related species M. agrestis and M. nivalis. Up to 15 different partial sequences of the SRY gene were found in M. cabrerae. Southern blots suggest that most of the extra copies of SRY are X-linked. One of the copies observed only in males has a sequence identical to that of the SRY gene in M. agrestis and may represent a functional copy of the gene in this species. The rest are probably nonfunctional pseudogenes.

Abstract: 1. Microtus cabrerae is an Iberian endemic vole species with specific adaptations to the subhumid mediterranean climate. Its living populations are under a regressive trend. The earliest known records of Microtus cabrerae date from the late Middle Pleistocene, and it originated from Microtus brecciensis. 2. We describe changes in the geographic distribution of Microtus cabrerae throughout its history based on its palaeontological record, and to link them to environmental changes that have taken place since the appearance of Microtus cabrerae. 3. A series of successive chronological intervals comprising the recorded existence of the species was established, so that the majority of the published fossil records of Microtus cabrerae could be used for analysis. For each interval, a map with the inferred distribution of the species was created. The maps were used to establishing variations in the species’ distribution through time. Biogeographic history of Microtus cabrerae 2 4. A first regression in the extent of the distribution of Microtus cabrerae took place in Marine Isotope Stage 2, when the species abandoned south-eastern France and central Spain, where it had been present since the beginning of the Late Pleistocene. This range contraction was probably due to the global decline in temperatures and rainfall that took place in this period. After a rapid recolonisation of most of the previously abandoned areas at the beginning of the Holocene, and a remarkable increase in records during the Neolithic, a new gradual decrease of records is observed from the Neolithic to the Roman period, intensifying from ca. 2000 years ago onwards, and ending with the final disappearance of the species from south-western France and north-eastern Iberia. This second decline is linked to the aridification of the Mediterranean entourage that started in the mid-Holocene and has been enhanced by human modification of the landscape. The species is shown to be sensitive to climate change.

Abstract: The extant Cabrera's vole, Microtus cabrerae, differs in morphology and evolutionary history from the other species of Microtus. This arvicoline has unique derived features in the cranium, mandible and dentition. Probably its most conspicuous features are its large size, the high skull in lateral view, the long and distally broad nasals, and the triangle shape of the anteroconid complex, with a marked labio-lingual asymmetry of the occlusal surface of the first lower molars. In this study, we propose a phylogenetic lineage that includes Cabrera's vole in what until now has been the Microtus subgenus Iberomys. Paleontological information and several life history traits support the elevation of Iberomys to the rank of genus. Genus Iberomys comprises species that have appeared in succession during the Quaternary: in the Early Pleistocene, the extinct I. huescarensis in the Middle Pleistocene, the extinct I. mediterraneus and in the Late Pleistocene, the extant I. cabrerae. Interestingly, the extant species shows several biological singularities, such as multiple polymorphic copies of the SRY male-specific gene in both males and females, and the lowest basal metabolic rate in relation to weight among arvicoline species. Likewise, its habitat requirement is unique among the Iberian arvicolines. Accordingly, the biological and paleontological data that we present in this work support the elevation of its taxonomic rank to that of genus. This study also suggests a modification of nomenclature: Microtus (Iberomys) brecciensis is replaced with I. mediterraneus and the common name of the extant M. (I.) cabrerae changed from ‘topillo’ to ‘iberon’ to improve conservation and protection actions.

Abstract: Conventional and microspread preparations of Microtus cabrerae spermatocytes were made to investigate the chromosomes of this species. Three different types of Y chromosomes, varying in size of the heterochromatic block, were observed; they were alike, however, in regard to synapsis, which was consistently absent. Our results suggest that the heterochromatic blocks are not involved in the lack of synapsis and that asynapsis is a cytological feature common to all species of the family Microtidae. In addition, the co-aligned configuration of the ends of the sex-chromosome axes of this species and the lack of silver-stainable threads or filaments connecting them suggest the existence of two mechanisms for association of the sex chromosomes during prophase I and metaphase I: attachment of the ends of both sex chromosome axes to the nuclear envelope and heterochromatin "stickiness."