Ortwin Elle

Species distribution models contribute to determine the effect of climate and interspecific interactions in moving hybrid zones

Climate is a major factor delimiting species’ distributions. However, biotic interactions may also be prominent in shaping geographical ranges, especially for parapatric species forming hybrid zones. Determining the relative effect of each factor and their interaction of the contact zone location has been difficult due to the lack of broad scale environmental data. Recent developments in species distribution modelling (SDM) now allow disentangling the relative contributions of climate and species’ interactions in hybrid zones and their responses to future climate change. We investigated the moving hybrid zone between the breeding ranges of two parapatric passerines in Europe. We conducted SDMs representing the climatic conditions during the breeding season. Our results show a large mismatch between the realized and potential distributions of the two species, suggesting that interspecific interactions, not climate, account for the present location of the contact zone. The SDM scenarios show that the southerly distributed species, Hippolais polyglotta, might lose large parts of its southern distribution under climate change, but a similar gain of novel habitat along the hybrid zone seems unlikely, because interactions with the other species (H. icterina) constrain its range expansion. Thus, whenever biotic interactions limit range expansion, species may become ‘trapped’ if range loss due to climate change is faster than the movement of the contact zone. An increasing number of moving hybrid zones are being reported, but the proximate causes of movement often remain unclear. In a global context of climate change, we call for more interest in their interactions with climate change.

Analytical limits of hybrid identification using genetic markers: an empirical and simulation study in Hippolais warblers

Hybridization is known to occur in a wide range of avian species, yet the rate and persistence of hybridization on populations is often hard to assess. Genotyping using variable genetic marker sets has become a common tool to identify hybrid individuals, however assignment outputs can differ depending on the marker set used. Here, we study hybrid assignment in two sibling Hippolais warblers, where hybrid assignment has shown to differ between SSR and AFLP markers. Simulation of heterospecific individuals as well as backcrosses (typed using SSR markers) reveals a rapid loss of assignment probability in higher backcross generations. However, the characterization of F1 hybrids was clearly distinguished from both parental taxa. The differences in marker sets are not contradictory but complementary. The rate of hybridization is lower than previously expected with AFLP markers but introgression might be long-lasting. This could be either due to differences in power of the marker systems used or due to non-neutral variation covered by AFLP but not SSR markers. We call for more attention to be paid regarding the potential limits of classical marker systems to investigate hybridization and its persistence in natural systems.

Cross-species utility of 22 microsatellite markers in the Melodious Warbler (Hippolais polyglotta)

Microsatellites are a valuable tool in the analysis of population genetic structure. Utilising microsatellite markers that were originally isolated from other species (cross-species amplification) can prove an efficient way, in terms of time and cost, to obtain markers for genetic studies. Here, 55 avian microsatellite primer pairs were tested for the cross-amplification in the Melodious Warbler (Hippolais polyglotta). Thirty-five markers amplified, of which 22 were polymorphic, displaying two to nine alleles in the 15 individuals genotyped. The 35 markers which amplified in the Melodious Warbler were tested in its sister species the Icterine Warbler (H. icterina). Twenty-four markers were amplified, 14 of which were polymorphic in the five H. icterina individuals genotyped. Thirteen loci were polymorphic in both species. The polymorphic loci identified are suitable for analysing the genetic population structure and assigning parentage.