Ancient Introgression in Mouse Lemurs (Microcebus: Cheirogaleidae) Explains 20 Years of Phylogenetic Uncertainty

Mouse lemurs (genus Microcebus) are a clade of approximately 26 named species of small, nocturnal primates endemic to Madagascar. The genus radiated one to ten million years ago and is morphologically cryptic, with most species having been named within the past 20 years largely based on phylogenetic analysis of short fragments of mitochondrial data. More recent work has been focused on revisiting species designations with autosomal nuclear data using more sophisticated statistical approaches. The order of speciation events in Microcebus remains contentious, particularly with regard to the placement of the M. ravelobensis clade. We investigated support for previous phylogenetic hypotheses based on available whole-genome assemblies from six species and an outgroup. We recovered over 4,000 one-to-one orthologs from these assemblies and used concatenation and coalescent species tree methods to evaluate if differences between previous studies were due to methodological differences or to limitations from too few loci. Observed gene tree discordance was high with patterns inconsistent with incomplete lineage sorting alone. Therefore, we estimated phylogenetic networks to investigate ancient introgression events that may explain observed gene tree distributions and previous phylogenetic conflicts. A network model, invoking some role for introgressive hybridization in the early evolution of Microcebus, better characterizes phylogenetic relationships than does any binary species tree. Our results provide insights into the biogeographic history of a threatened and diverse group of primates while also highlighting an important role for phylogenetic network methods in resolving cases of phylogenetic uncertainty.