Reproductive Isolation between Host Races of Phytomyza glabricola on Ilex coriacea and I. glabra

Abstract

Recently diverged taxa often show discordance in genetic divergence among genomic loci, where some loci show strong divergence and others show none at all. Genetic studies alone cannot distinguish among the possible mechanisms but experimental studies on other aspects of divergence may provide guidance in the inference of causes of observed discordances. In this study, we used no-choice mating trials to test for the presence of reproductive isolation between host races of the leaf-mining fly, Phytomyza glabricola on its two holly host species, Ilex coriacea and I. glabra. These trials inform our effort to determine the cause of significant differences in the degree of divergence of nuclear and mitochondrial loci of flies collected from the two host plants. We present evidence of reproductive isolation between host races in a controlled greenhouse setting: significantly more mate pairs consisting of flies from the same host plant species produced offspring than inter-host mate pairs, which produced no offspring. We also tested whether the presence of the natal or non-natal host plant affects reproductive success. Flies collected from I. coriacea were more likely to produce offspring when in the presence of the natal host, whereas the presence or absence of either the natal or non-natal host had no effect on flies collected from I. glabra. The results indicate discordant patterns of nuclear and mitochondrial divergence among host races of P. glabricola are likely due to incomplete lineage sorting, and the host races may be well on their way to becoming biological species.

Revisiting the particular role of host shifts in initiating insect speciation

“…At least one reproductive isolating barrier, immigrant inviability, is related to host use (Herbert et al. ), and a recent genome scan suggests that several loci have experienced host‐associated selection (Hebert et al. ).…”

“…Though a lack of genetic structure among mitochondrial sequences suggests that this shift has been relatively recent (Scheffer and Hawthorne ), attempts to produce hybrid offspring have failed (Hebert et al. ), so it remains possible that these flies are already different “species” that could have differentiated before one of them shifted to a new host (Fig. C).…”

Evidence for ecological speciation via a host shift in the holly leaf miner,Phytomyza glabricola(Diptera: Agromyzidae)

“…We have previously demonstrated that hybridization of flies from the respective host plants has a 100% failure rate in the laboratory, indicating some measure of reproductive isolation that may carry into the field (Hebert et al. ). However, no host‐associated divergence in mitochondrial sequences has been found (Scheffer and Hawthorne ), suggesting that the nuclear sequence and AFLP divergence are recent or that some gene flow may occur in the field.…”

“…Despite these phenological differences, adult P. glabricola from both hosts emerge in synchrony once a year in mid‐January to mid‐February, potentially allowing mating of flies from the two host plant species (Scheffer ), although laboratory mating experiments produced no adult fly hybrid offspring (Hebert et al. ).…”

“…Phytomyza glabricola feeding on I. coriacea ("coriaceaflies") have a development time of 9-10 months with a single generation a year (Kulp 1968;Al-Siyabi and Shetlar 1998;Scheffer 2002;Scheffer and Hawthorne 2007). Despite these phenological differences, adult P. glabricola from both hosts emerge in synchrony once a year in mid-January to mid-February, potentially allowing mating of flies from the two host plant species (Scheffer 2002), although laboratory mating experiments produced no adult fly hybrid offspring (Hebert et al 2013).…”