Patterns of reproductive isolation in a haplodiploid – strong post‐mating, prezygotic barriers among three forms of a social spider mite

Abstract

In speciation research, much attention is paid to the evolution of reproductive barriers, preventing diverging groups from hybridizing back into one gene pool. The prevalent view is that reproductive barriers evolve gradually as a by-product of genetic changes accumulated by natural selection and genetic drift in groups that are segregated spatially and/or temporally. Reproductive barriers, however, can also be reinforced by natural selection against maladaptive hybridization. These mutually compatible theories are both empirically supported by studies, analysing relationships between intensity of reproductive isolation and genetic distance in sympatric taxa and allopatric taxa. Here, we present the - to our knowledge - first comparative study in a haplodiploid organism, the social spider mite Stigmaeopsis miscanthi, by measuring premating and post-mating, pre- and post-zygotic components of reproductive isolation, using three recently diverged forms of the mite that partly overlap in home range. We carried out cross-experiments and measured genetic distances (mitochondrial DNA and nuclear DNA) among parapatric and allopatric populations of the three forms. Our results show that the three forms are reproductively isolated, despite the absence of premating barriers, and that the post-mating, prezygotic component contributes most to reproductive isolation. As expected, the strength of post-mating reproductive barriers positively correlated with genetic distance. We did not find a clear pattern of prezygotic barriers evolving faster in parapatry than in allopatry, although one form did show a trend in line with the ecological and behavioural relationships between the forms. Our study advocates the versatility of haplodiploid animals for investigating the evolution of reproductive barriers.

Phylogeography of lethal male fighting in a social spider mite

“…DNA was extracted from a single female mite from each population, which was stored in 100% ethanol or acetone in a microtube, by using Wizard Genomic DNA Purification Kit (Promega, Madison, WI, USA), PrepMan Ultra Sample Preparation Reagent (Applied Biosystems, Foster City, CA, USA), or QIAamp DNA micro kit (Qiagen K. K., Tokyo, Japan). To amplify the fragment of parasodium channel gene, polymerase chain reaction (PCR) is carried out using primers given in Supporting Information Table (Sato et al, ) in a 20 μl reaction mixture containing 1 μl of DNA sample, 2 μl of 10× Ex Taq buffer (20 mM mg 2+ plus; Takara Bio Inc., Otsu, Shiga, Japan), 0.2 μl of TaKaRa Ex Taq (5 U/μl; Takara Bio Inc.), 1.6 μl of dNTP mix (2.5 mM each; Takara Bio Inc.), 1 μl of each primer (10 pmol/ul each), and 13.2 μl of ddH 2 O. PCR cycling conditions were 2 min at 94°C, followed by 38 cycles of 30 s at 94°C, 30 s at 53°C, and 1 min at 72°C, and a final extension at 72°C for 10 min. To amplify the fragment of mtCOI gene, PCR is carried out using primers given in Supporting Information Table (Matsuda, Morishita, Hinomoto, & Gotoh, ) in a 10 μl reaction mixture containing 1 μl of DNA sample, 1 μl of 10× Ex Taq buffer (20 mM mg 2+ plus; Takara Bio Inc.), 0.05 μl of TaKaRa Ex Taq (5 U/μl; Takara Bio Inc.), 0.8 μl of dNTP mix (2.5 mM each; Takara Bio Inc.), 0.5 μl of each primer (10 pmol/μl each), and 6.35 μl of ddH 2 O. PCR cycling conditions were 4 min at 94°C, followed by 35 cycles of 1 min at 94°C, 1 min at 45°C, and 1.5 min at 72°C, and a final extension at 72°C for 10 min.…”

“…In some samples, the fragment was not amplified. Therefore, in the samples, PCR is carried out using another primers in Table (Sato et al, ) in a 50 μl reaction mixture containing 1 μl of DNA sample, 25 μl of Premix Ex Taq (1.25 U/25 μl; Takara Bio Inc.), and 24 μl of ddH 2 O, and PCR conditions were 1 min at 94°C, followed by 35 cycles of 10 s at 98°C, 30 s at 50°C, and 1 min at 72°C without a final extension. PCR products were purified using QIA quick PCR Purification Kit (QIAGEN) or ExoSAP‐IT PCR Product Cleanup (Affymetrix Inc., Cleveland, OH, USA), and the purified products were sequenced using ABI BigDye Terminator ver.…”

“…A part of the samples was sequenced by an external service (FASMAC Co., Ltd., Atsugi, Kanagawa, Japan). The sequences data of 12 out of 47 S. miscanthi populations and one of two Stigmaeopsis longus populations were obtained from previous studies (Sato et al, ) (GenBank accession numbers: –, –), since the DNA preparation and sequences were carried out in the same way, the same populations and the same period. The sequence data of 35 S. miscanthi populations and one Stigmaeopsis longus population obtained in this study have been submitted to GenBank database under accession number –.…”

“…Populations with low and high aggressiveness are discriminated as LW and HG forms, because of their behavioral, ecological, and morphological differences (Saito & Sahara, ; Saito, Sakagami, & Sahara, ; Yano, Saito, Chittenden, & Sato, ), their molecular phylogeny (Ito & Fukuda, ; Sakagami, Saito, Kongchuensin, & Sahara, ; Sakamoto et al, ) and also incomplete but strong reproductive isolation between them (Sato, Breeuwer, Egas, & Sabelis, ; Sato, Saito, & Mori, ). Recently, populations with mild male–male aggressiveness and intermediate male weapon morph were found (Sato, Egas, et al, ), yet their ecological and phylogenetic relationships with the LW and HG forms are unclear (Sato et al, ). Using a different set of sampled populations, Sakamoto et al, () identified a third clade in S. miscanthi, which consisted of a population collected from Fuzhou, China, by phylogenetic analysis based on COI and 18S–28S genes.…”

Successful mating and hybridisation in two closely related flatworm species despite significant differences in reproductive morphology and behaviour

“…They are 65 usually isolated from interbreeding with other species by reproductive barriers, though in 66 some cases they remain capable of producing hybrid offspring with closely related species. 67 Accordingly, an important step for the origin and maintenance of species is the evolution of 68 reproductive barriers, which are usually split into prezygotic and postzygotic barriers (Butlin 69 et al 2012;Ostevik et al 2016;Lackey and Boughman 2017;Sato et al 2018). While 70 prezygotic barriers involve the prevention of zygote formation, postzygotic barriers lead to 71 zygote mortality, or inviable or sterile hybrid offspring that are unable to pass on their genes.…”