Résumé
Habitat fragmentation is a major cause of biodiversity loss, responsible
for an alteration of intraspecific patterns of neutral genetic diversity
and structure. Although neutral genetic variation can be informative for
demographic inferences, it may be a poor predictor of adaptive genetic
diversity and thus of the consequences of habitat fragmentation on
selective evolutionary processes. In this context, we contrasted patterns
of genetic diversity and structure of neutral loci (microsatellites) and
immune genes (i.e., toll-like receptors) in an understorey bird species,
the wedge-billed woodcreeper Glyphorynchus spirurus. The objectives were
(1) to investigate forest fragmentation effects on population genetic
diversity, (2) to disentangle the relative role of demography (genetic
drift and migration) and selection, and (3) to assess whether
immunogenetic patterns could be associated with variation of ectoparasite
(i.e., ticks) pressures. Our results revealed an erosion of neutral
genetic diversity and a substantial genetic differentiation among
fragmented populations, resulting from a decrease in landscape
connectivity and leading to the divergence of distinct genetic pools at a
small spatial scale. Patterns of genetic diversity observed for TLR4 and
TLR5 were concordant with neutral genetic patterns, whereas those observed
for TLR3 and TLR21 were discordant. This result underlines that the
dominant evolutionary force shaping immunogenetic diversity (genetic drift
vs. selection) may be different depending on loci considered. Finally,
tick prevalence was higher in fragmented environments. We discussed the
hypothesis that pathogen selective pressures may contribute to maintain
adaptive genetic diversity despite the negative demographic effect of
habitat fragmentation on neutral genetic diversity.