Are Eurosta solidaginis on Solidago rugosa a divergent host-associated race?

The ball-gall fly Eurosta solidaginis is considered a classic example of host-race formation

in herbivorous insects, with host-associated races evolving at least twice, including

the well-known pair on Solidago altissima and S. gigantea. Yet E. solidaginis has been

occasionally observed galling other Solidago species. Here, we explore the origins of E.

solidaginis on Solidago rugosa. We hypothesize that flies associated with S. rugosa are

derived from the S. altissima host-race, and ask whether S. rugosa-associated flies have

initiated host-race formation. We compared genetic variation among seventeen E. solidaginis

populations collected from S. rugosa, S. altissima, and S. gigantea, in an adjacent COI/

COII region of the mitochondrial genome. Across the study area, E. solidaginis flies from

S. rugosa were as diverged from S. altissima-flies as they were from S. gigantea-flies (pairwise

ΦPT 0.075 and 0.78 respectively) but S. altissima and S. gigantea-flies appeared considerably

less diverged (0.002). This pattern was driven by the majority of flies, regardless

of host-plant, sharing the same haplotype across the study area. However, we detected several

site/region-specific haplotypes, not shared among host species. At the local site scale

we were able to distinguish S. gigantea-associated fly haplotypes from either S. altissima

or S. rugosa haplotypes, but the majority of S. altissima and S. rugosa-flies shared the

same haplotype locally. These patterns of haplotype diversity support existing evidence

of host-associated divergence in S. altissima- and S. gigantea-associated flies, and suggest

that S. rugosa flies are either the same host-race as, or are recently derived from,

S. altissima flies. Successful development on S. rugosa and the use of S. rugosa in the

absence of use on sympatric S. altissima suggests that E. solidaginis either has a single

oligophagous race using both S. altissima and S. rugosa or is in the earliest stages of hostassociated

differentiation.