Constricted range and limited genetic variation in Reig's Grass Mouse Akodon reigi (Cricetidae: Sigmodontinae) in the Southern Campos

We present an updated distribution of Akodon reigi , a sigmodontine endemic to Campos in southern South America, and explore its habitat of occurrence and genetic diversity. Our findings reveal new occurrences of A. reigi extending approximately 110 km northwest and 120 km northeast beyond previously known limits. Contrary to previous assumptions of close association with forest formations, recent sightings suggest a degree of habitat plasticity within the species, with specimens found in shrubby/herbaceous phytophysiognomies. Fifty-three individuals sequenced for Cytochrome b (cyt b ; 806 bp) and the first exon of Interphotoreceptor Retinoid Binding Protein (IRBP; 408 bp) showed low variability. Eleven distinct cyt b haplotypes were identified. The largest haplotype richness was observed in a locality in the Sul-Riograndense Shield, a region that could represent a center of the species' genetic diversification. No nucleotide variation was observed for IRBP. No significant indications of population expansion or contraction were detected. The relatively small range, the low genetic diversity, and the current condition of habitat stress due to floods related to global climate change draw attention to the species’ conservation.