Hybrid zone between Ctenomys lami and C. minutus: habitat alterations influence the evolutionary history of two burrowing rodent species of southern Brazil

Abstract

Hybridization events provide insights into central questions in evolutionary biology regarding the role of reproductive isolation in speciation mechanisms.  Because of that, there is increasing interest in understanding the patterns and processes underlying hybrid zones.  The sister species Ctenomys minutus (2n = 48a) and Ctenomys lami (2n = 56) are burrowing rodents endemic to the southern Brazilian coastal plain and collectively exhibit high karyotype polymorphism.  We analyzed chromosomal and mitochondrial DNA (mtDNA) variation to assess their evolutionary history and confirm and characterize an interspecific hybrid zone between the species.  Despite the absence of reciprocal monophyly of mtDNA lineages, C. minutus and C. lami maintain distinct geographical distributions as well as cytogenetic and morphological differences, suggesting that these incipient species are in the early stages of evolutionary differentiation.  The 19 hybrid individuals we include in this analysis show intermediate karyotypic forms between the parental types.  These admixed individuals all shared the same mtDNA haplotype that is otherwise only found in C. minutus.  Although C. minutus and C. lami are incipient species, their unique differences require distinct conservation efforts.  Conservation efforts should focus on maintaining the integrity of pure populations of both species and minimizing anthropogenically induced hybridization.