title: Sex- and species-biased gene flow in a spotted eagle hybrid zone.
abstract: BACKGROUND
  Recent theoretical and empirical work points toward a significant role for sex-chromosome linked genes in the evolution of traits that induce reproductive isolation and for traits that evolve under influence of sexual selection. Empirical studies including recently diverged (Pleistocene), short-lived avian species pairs with short generation times have found that introgression occurs on the autosomes but not on the Z-chromosome. Here we study genetic differentiation and gene flow in the long-lived greater spotted eagle (Aquila clanga) and lesser spotted eagle (A. pomarina), two species with comparatively long generation times.
  
  
  RESULTS
  Our data suggest that there is a directional bias in migration rates between hybridizing spotted eagles in eastern Europe. We find that a model including post divergence gene flow fits our data best for both autosomal and Z-chromosome linked loci but, for the Z-chromosome, the rate is reduced in the direction from A. pomarina to A. clanga.
  
  
  CONCLUSIONS
  The fact that some introgression still occurs on the Z-chromosome between these species suggests that the differentiation process is in a more premature phase in our study system than in previously studied avian species pairs and that could be explained by a shorter divergence time and/or a longer average generation time in the spotted eagles. The results are in agreement with field observations and provide further insight into the role of sex-linked loci for the build-up of barriers to gene flow among diverging populations and species.
authors.affiliation: Department of Organismic and Evolutionary Biology, Museum of Comparative Zoology, Harvard University, 26 Oxford Street, Cambridge, MA 02138, USA.
authors.forename: Niclas
authors.initials: N
authors.lastname: Backström
authors.forename: Ulo
authors.initials: U
authors.lastname: Väli
mesh_terms: D000818:Animals; D020307:Eagles / Q000235:genetics*; D005061:Europe, Eastern; D005260:Female; D051456:Gene Flow; D049810:Genetic Speciation; D008297:Male; D008957:Models, Genetic; D012730:Sex Chromosomes; D013045:Species Specificity