Molecular heterogeneity in major urinary proteins of Mus musculus subspecies: potential candidates involved in speciation

Abstract

When hybridisation carries a cost, natural selection is predicted to favour evolution of traits that allow assortative mating (reinforcement). Incipient speciation between the two European house mouse subspecies, Mus musculus domesticus and M.m.musculus, sharing a hybrid zone, provides an opportunity to understand evolution of assortative mating at a molecular level. Mouse urine odours allow subspecific mate discrimination, with assortative preferences evident in the hybrid zone but not in allopatry. Here we assess the potential of MUPs (major urinary proteins) as candidates for signal divergence by comparing MUP expression in urine samples from the Danish hybrid zone border (contact) and from allopatric populations. Mass spectrometric characterisation identified novel MUPs in both subspecies involving mostly new combinations of amino acid changes previously observed in M.m.domesticus. The subspecies expressed distinct MUP signatures, with most MUPs expressed by only one subspecies. Expression of at least eight MUPs showed significant subspecies divergence both in allopatry and contact zone. Another seven MUPs showed divergence in expression between the subspecies only in the contact zone, consistent with divergence by reinforcement. These proteins are candidates for the semiochemical barrier to hybridisation, providing an opportunity to characterise the nature and evolution of a putative species recognition signal.

Evolutionary patterns of major urinary protein scent signals in house mice and relatives

“…Male M. m. musculus express Mup20 at high levels (Figure ) but, as mentioned above, express a Mup3 ‐like gene at extremely low levels. In both M. m. musculus and M. m. domesticus expression of Mup20 is strongly male‐biased, consistent with previous reports (Hurst et al, ; Roberts et al, ).…”

“…Investment in overall levels of Mup expression varies among species and sexes (ANOVA, species: F 7,89 = 10.05, p = 5.25e−9; sex: F 1,89 = 47.13, p = 1.17e−9, species*sex: F 7,89 = 2.8, p = .012). Across species, males show higher levels of Mup gene expression (Figure ), consistent with patterns previously reported in subspecies of M. musculus (Hurst et al, ; Stopková, Stopka, Janotova, & Jedelsky, ). Overall levels of expression vary across species, though expression levels in males and females are correlated (linear regression, t 6 = 3.16, r 2 = .63, p = .019).…”

“…Protein analysis of the same M. m. musculus population by the same group, however, report variability in the number of distinct protein bands and proteins identified from male urine (Enk et al, ; Thoß, Luzynski, Ante, Miller, & Penn, ). Indeed, protein heterogeneity among M. m. musculus has been further reported by other groups (Hurst et al, ) highlighting that MUP diversity appears to be widespread among M. musculus subspecies.…”

“…Information content of signals is determined by the distribution of variation among individuals within and between groups (Dale, 2006;Tibbetts, Mullen, & Dale, 2017). Individual identity information is encoded by the diverse central MUPs, genes that differ among individuals in both sequence and relative expression patterns (Hurst et al, 2001(Hurst et al, , 2017Sheehan et al, 2016). Sex is encoded by the differential expression of Mup3 and Mup20.…”

Population fragmentation drives up genetic diversity in signals of individual identity

“…There are a number of examples of signal characteristics varying with geography, including in chimpanzee calls (Mitani et al ), in major urinary protein expression among subspecies of house mice (Hurst et al , Sheehan et al ), in human faces (Guo et al ), and in intraspecific bird song dialects (Baker and Cunningham ). While this geographic diversity can develop under a number of processes, our model predicts such variation in fragmented populations of species in which there is temporal separation of mate assessment and mating.…”