Mediterranean Pine Vole, Microtus duodecimcostatus: A Paradigm of an Opportunistic Breeder Lao Pérez, Miguel Mohamed Mahmoud Massoud, Diaa Fawzi Real, Francisca M. Hurtado, Alicia Ortega Sánchez, Esperanza Burgos Poyatos, Miguel Jiménez Medina, Rafael Barrionuevo Jiménez, Francisco Javier Seasonal breeding Testis regression Microtus duodecimcostatus Opportunistic breeding Most mammalian species of the temperate zones of the Earth reproduce seasonally, existing a non-breeding period in which the gonads of both sexes undergo functional regression. It is widely accepted that photoperiod is the principal environmental cue controlling these seasonal changes, although several exceptions have been described in other mammalian species in which breeding depends on cues such as food or water availability. We studied the circannual reproductive cycle in males of the Mediterranean pine vole, Microtus duodecimcostatus, in the Southeastern Iberian Peninsula. Morphological, hormonal, functional, molecular and transcriptomic analyses were performed. As reported for populations of other species from the same geographic area, male voles captured in wastelands underwent seasonal testis regression in summer whereas, surprisingly, those living either in close poplar plantations or in our animal house reproduced throughout the year, showing that it is the microenvironment of a particular vole subpopulation what determines its reproductive status and that these animals are pure opportunistic, photoperiod-independent breeders. In addition, we show that several molecular pathways, including MAPK, are deregulated and that the testicular “immune privilege” is lost in the inactive testes, providing novel mechanisms linking seasonal testosterone reduction and testis regression. 2021-06-07T07:53:43Z 2021-06-07T07:53:43Z 2021 info:eu-repo/semantics/article Lao-Pérez, M.; Massoud, D.; Real, F.M.; Hurtado, A.; Ortega, E.; Burgos, M.; Jiménez, R.; Barrionuevo, F.J. Mediterranean Pine Vole, Microtus duodecimcostatus: A Paradigm of an Opportunistic Breeder. Animals 2021, 11, 1639. https://doi.org/10.3390/ ani11061639 http://hdl.handle.net/10481/69006 10.3390/ani11061639 eng http://creativecommons.org/licenses/by/3.0/es/ info:eu-repo/semantics/openAccess Atribución 3.0 España MDPI