The role of allochrony in speciation

Taxa harboring high levels of standing variation may be more likely to adapt to rapid environmental shifts and experience ecological speciation. Here, we characterize geographic and host‐related differentiation for 10,241 single nucleotide polymorphisms in Rhagoletis pomonella fruit flies to infer whether standing genetic variation in adult eclosion time in the ancestral hawthorn (Crataegus spp.)‐infesting host race, as opposed to new mutations, contributed substantially to its recent shift to earlier fruiting apple (Malus domestica). Allele frequency differences associated with early vs. late eclosion time within each host race were significantly related to geographic genetic variation and host race differentiation across four sites, arrayed from north to south along a 430‐km transect, where the host races co‐occur in sympatry in the Midwest United States. Host fruiting phenology is clinal, with both apple and hawthorn trees fruiting earlier in the North and later in the South. Thus, we expected alleles associated with earlier eclosion to be at higher frequencies in northern populations. This pattern was observed in the hawthorn race across all four populations; however, allele frequency patterns in the apple race were more complex. Despite the generally earlier eclosion timing of apple flies and corresponding apple fruiting phenology, alleles on chromosomes 2 and 3 associated with earlier emergence were paradoxically at lower frequency in the apple than hawthorn host race across all four sympatric sites. However, loci on chromosome 1 did show higher frequencies of early eclosion‐associated alleles in the apple than hawthorn host race at the two southern sites, potentially accounting for their earlier eclosion phenotype. Thus, although extensive clinal genetic variation in the ancestral hawthorn race exists and contributed to the host shift to apple, further study is needed to resolve details of how this standing variation was selected to generate earlier eclosing apple fly populations in the North.

Section: Con Clus Ionmentioning

confidence: 99%

Standing geographic variation in eclosion time and the genomics of host race formation in Rhagoletis pomonella fruit flies

Doellman

Egan

Ragland

et al. 2018

Section: Discussionmentioning

confidence: 99%

“…Differences in breeding time can be interpreted as an alternate to spatial differentiation, or as a type of ecological differentiation that warrants further attention, as examples in the literature remain sparse (Taylor & Friesen, ). Peristenus specialization on different generations of Lygus may have led to temporal assortative mating limiting gene flow, equating to allopatric populations separated by temporal rather than physical barriers (Taylor & Friesen, ). Without knowing the full distribution range and biogeographic history of these two Peristenus species, it is difficult to determine whether temporal separation was the cause of the speciation event or the result of niche partitioning in the form of secondary reinforcement when they came into secondary contact in Alberta.…”

Section: Discussionmentioning

confidence: 99%

“…Adaptation to divergent host plants leads to an accumulation of multiple reproductive barriers, ultimately resulting in the separation and formation of new species (Dres & Mallet, ; Forbes et al., ). The presence of HAD is often associated with a temporal component, where temporal divergence in the breeding time over timescales ranging from days, seasons, or even years (Taylor & Friesen, ). Temporal isolation can contribute to divergence alone or concurrently with traits such as host preference to reinforce divergence along the speciation continuum (Egan et al., ; Feder et al., ; Taylor & Friesen, ).…”

Section: Introductionmentioning

confidence: 99%

“…The presence of HAD is often associated with a temporal component, where temporal divergence in the breeding time over timescales ranging from days, seasons, or even years (Taylor & Friesen, ). Temporal isolation can contribute to divergence alone or concurrently with traits such as host preference to reinforce divergence along the speciation continuum (Egan et al., ; Feder et al., ; Taylor & Friesen, ). Although allopatric populations are often defined by spatial differentiation, populations with overlapping distributions and phenological differences can also be argued as allopatric, but on a temporal scale (Taylor & Friesen, ).…”

Section: Introductionmentioning

confidence: 99%

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Habitat or temporal isolation: Unraveling herbivore–parasitoid speciation patterns using double digest RADseq

Zhang

Bass

Fernández

et al. 2018

Ecological speciation is often observed in phytophagous insects and their parasitoids due to divergent selection caused by host‐associated or temporal differences. Most previous studies have utilized limited genetic markers or distantly related species to look for reproductive barriers of speciation. In our study, we focus on closely related species of Lygus bugs and two sister species of Peristenus parasitoid wasps. Using mitochondrial DNA COI and genomewide SNPs generated using ddRADseq, we tested for potential effects of host‐associated differentiation (HAD) or temporal isolation in this system. While three species of Lygus are clearly delineated with both COI and SNPs, no evidence of HAD or temporal differentiation was detected. Two Peristenus sister species were supported by both sets of markers and separated temporally, with P. mellipes emerging early in June and attacking the first generation of Lygus, while P. howardi emerging later in August and attacking the second generation of their hosts. This is one of the few studies to examine closely related hosts and parasitoids to examine drivers of diversification. Given the results of this study, the Lygus‐Peristenus system demonstrates temporal isolation as a potential barrier to reproductive isolation for parasitoids, which could indicate higher parasitoid diversity in regions of multivoltine hosts. This study also demonstrates that incorporating systematics improves studies of parasitoid speciation, particularly by obtaining accurate host records through rearing, carefully delimiting cryptic species and examining population‐level differences with genomic‐scale data among closely related taxa.

Male courtship preference during seasonal sympatry may maintain population divergence

Kimmitt

Dietz

Reichard

et al. 2018

Animal migration can lead to a population distribution known as seasonal sympatry, in which closely‐related migrant and resident populations of the same species co‐occur in sympatry during part of the year, but are otherwise allopatric. During seasonal sympatry in early spring, residents may initiate reproduction before migrants depart, presenting an opportunity for gene flow. Differences in reproductive timing between migrant and resident populations may favor residents that exhibit preferences for potential mates of similar migratory behavior and reproductive timing, thus maintaining population divergence. We studied dark‐eyed juncos (Junco hyemalis), a songbird that exhibits seasonal sympatry. We conducted simulated courtship interactions in which we presented free‐living resident males with either a caged migrant or resident female and quantified courtship behavior prior to the departure of the migrants. We found that resident males preferred to court resident females: they sang more short‐range songs and exhibited more visual displays associated with courtship when presented with resident females. We conclude that males distinguish between migrant and resident females during seasonal sympatry when the risk of interacting with non‐reproductive, migrant females is high. Male mate choice in seasonal sympatry is likely adaptive for male reproductive success. As a secondary effect, male mating preference could act to maintain or promote divergence between populations that differ in migratory strategy.