Data from: Weak geographical structure in sperm morphology across the range of two willow warbler Phylloscopus trochilus subspecies in Scandinavia

Sperm morphology is highly diversified among species and at higher
taxonomic levels. In birds, there is also increasing evidence of
geographical differentiation in sperm traits within species, especially in
those with strong sperm competition. Geographical divergences in sperm
traits might imply the formation of a reproductive barrier in a speciation
process. Here we study sperm morphology variation of willow warblers
Phylloscopus trochilus in a geographical context in Scandinavia, across
the range of two subspecies that are differentiated in certain genetic
markers, morphology and migratory routes. We describe geographical
patterns in genotypes (two previously described single-nucleotide
polymorphism (SNP) markers and four polymorphic microsatellites); in wing
length, tarsus length and body mass; and in sperm traits of 330 male
willow warblers sampled at 33 localities across Norway (58°N–69°N). Birds
were on average larger and longer-winged in the north (spp. acredula) than
in the south (spp. trochilus), and showed a sigmoid change in the SNP
allele frequencies and body morphology around 65°N. We found no evidence
of genetic structuring in the microsatellites. There was no geographical
variation in sperm traits across Norway, except that sperm heads were on
average longer in the south. Sperm head length was also associated with
the two SNP markers, with longer sperm heads for the southern alleles, and
midpiece length was weakly associated with one of the SNP markers. Similar
among-male variances in total sperm length among the 33 sampling sites
indicate uniform levels of sperm competition across Norway. We conclude
that sperm morphology remains a rather undifferentiated trait between the
two willow warbler subspecies in Scandinavia, which is consistent with a
pattern of a shallow genetic divergence. This indicates that sperm
morphology is not a reproductive barrier maintaining the narrow hybrid
zone.