Norrbom, A. L. | bioRxiv Science

bioRxiv2022

Stronger sexual dimorphism in fruit flies may be favored when congeners are present and females actively search for mates

Why are some species sexually dimorphic while other closely related species are not? When the degree of sexual dimorphism varies within a genus, an integrative phylogenetic approach may help reveal underlying patterns favoring the evolution of dimorphism. While all female flies in genus Strauzia - a genus of true fruit flies - share a multiply-banded wing pattern, males of four species have patterns wherein bands have "coalesced" into a continuous dark streak across much of the wing. We find that the origin of coalesced male wing patterns and pronounced differences in male wing shape correlate with the presumed origin of host plant sharing in this genus. A survey of North American Tephritidae finds just three other genera with specialist species that share host plants. Each has one or more congeners with wing patterns unusual for that genus, and just one genus, Eutreta, has those unusual wing patterns only in the male sex. Eutreta is also the only other genus among this subset wherein, like Strauzia, males hold territories while females search for mates. Sharing the same hosts may result in reproductive character displacement, and when coupled with a biology wherein females actively search for males, may specifically favor sexually dimorphic wing patterns.

evolutionary biology↗

bioRxiv2020

Host shifting and host sharing in a genus of specialist flies diversifying alongside their sunflower hosts

Congeneric parasites are unlikely to specialize on the same tissues of the same host species, likely because of strong multifarious selection against niche overlap. Exceptions where multiple congeneric species overlap on the same tissues may therefore reveal important insights into the ecological factors underlying the origins and maintenance of diversity. Larvae of sunflower maggot flies in genus Strauzia feed on the pith of plants in the family Asteraceae. Although Strauzia tend to be host specialists, some species overlap in their host use. To resolve the origins of host sharing among these specialist flies, we used reduced representation genomic sequencing to infer the first multi-locus phylogeny of genus Strauzia. Our results show that Helianthus tuberosus and Helianthus grosseserratus each host three different fly species, and that the flies co-occurring on a host are not one anothers closest relatives. Though this pattern implies that host sharing is most likely the result of host shifts, these may not be host shifts in the conventional sense of an insect moving onto an entirely new plant. Many hosts of Strauzia belong to a young (1-2 MYA) clade of perennial sunflowers noted for their frequent introgression and hybrid speciation events. In at least one case, flies may have converged upon a host after their respective ancestral host plants hybridized to form a new sunflower species (H. tuberosus). Broadly, we suggest that rapid and recent adaptive introgression and speciation in this group of plants may have instigated the diversification of their phytophagous fly associates, including the convergence of >1 species onto the same shared host plants.

evolutionary biology↗