Abstract
The plausibility of sympatric speciation is still debated despite increasing evidence, such as host races in insects. This speciation process may be occurring in the case of the two phenological forms of the obligatorily myrmecophilous Phengaris arion. The main goal of our research was to study the nature and causes of difference between these forms focusing primarily on the incipient speciation via host races. Molecular analyses based on highly variable microsatellites together with Wolbachia screening, male genitalia morphometrics and host ant studies were carried out on four syntopic sample pairs. Our results show that the two phenological forms of P. arion may meet the criteria for host plant races. They coexist in sympatry in certain parts of the species range which is allowed by the adaptation to the distinct phenology of the host plants. Negative selection acts against the intermediate individuals which are on the wing in the inappropriate time frame. Thus, disruptive selection affects and produces bimodal distributions of phenotypes. However, the phenology of food plants is not entirely distinct and fluctuates year by year. Therefore, the two forms can exchange genes occasionally depending on the length of the time slot when they can meet with each other. Consequently, the reproductive isolation could not be completed and the existence of the two arion forms may represent only an incipient stage of sympatric speciation. It is also clear that Wolbachia is likely not a driver of sympatric speciation in this case.
Highlights
Speciation is considered as a more or less continuous process starting from polymorphisms within species through the emergence of intermediate forms with increasing reproductive isolation up to the formation of new species
On our sampling sites the phenology of the host plants is usually distinct which is followed by the flight period of P. arion, i.e. there are two more or less separated flight peaks adjusted to the flowering peaks of the food plants
Our results show that the two phenological forms of P. arion seem to meet all criteria of host plant races, spatial replicability should be tested on a larger geographical scale
Summary
Speciation is considered as a more or less continuous process starting from polymorphisms within species through the emergence of intermediate forms with increasing reproductive isolation up to the formation of new species. Substantial indications, exist that sympatric speciation may have a significant role in the evolution of insects (Berlocher and Feder 2002; Bolnick and Fitzpatrick 2007; Coyne 2007; Drès and Mallet 2002; Via 2001). Hostassociated biotypes, including host races in plant-feeding insects, have often been used as evidence for the plausibility of sympatric speciation representing its incipient stage 1% per generation and (v) are spatially and temporally replicable, i.e. are more genetically differentiated from populations on another host in sympatry (and at the same time) than at least some geographically distant populations on the same host (Drès and Mallet 2002) Host races are defined as sets of populations that (i) coexist in sympatry in at least part of their range, (ii) use different host taxa and consist of individuals that exhibit ‘host fidelity’, i.e. are associated with a particular host, (iii) are genetically differentiated at more than one locus, (iv) regularly exchange genes at a rate of more than ca. 1% per generation and (v) are spatially and temporally replicable, i.e. are more genetically differentiated from populations on another host in sympatry (and at the same time) than at least some geographically distant populations on the same host (Drès and Mallet 2002)
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