Patterns of reproductive isolation in a haplodiploid mite, <i>Amphitetranychus viennensis</i>:Prezygotic isolation, hybrid inviability and hybrid sterility

BACKGROUND: Evolution of reproductive isolation is an important process, generating biodiversity and driving speciation. To better understand this process, it is necessary to investigate factors underlying reproductive isolation through various approaches but also in various taxa. Previous studies, mainly focusing on diploid animals, supported the prevalent view that reproductive barriers evolve gradually as a by-product of genetic changes accumulated by natural selection by showing a positive relationship between the degree of reproductive isolation and genetic distance. Haplodiploid animals are expected to generate additional insight into speciation, but few studies investigated the prevalent view in haplodiploid animals. In this study, we investigate whether the relationship also holds in a haplodiploid spider mite, Amphitetranychus viennensis (Zacher). RESULTS: We sampled seven populations of the mite in the Palaearctic region, measured their genetic distance (mtDNA) and carried out cross experiments with all combinations. We analyzed how lack of fertilization rate (as measure of prezygotic isolation) as well as hybrid inviability and hybrid sterility (as measures of postzygotic isolation) varies with genetic distance. We found that the degree of reproductive isolation varies among cross combinations, and that all three measures of reproductive isolation have a positive relationship with genetic distance. Based on the mtDNA marker, lack of fertilization rate, hybrid female inviability and hybrid female sterility were estimated to be nearly complete (99.0–99.9% barrier) at genetic distances of 0.475–0.657, 0.150–0.209 and 0.145–0.210, respectively. Besides, we found asymmetries in reproductive isolation. CONCLUSIONS: The prevalent view on the evolution of reproductive barriers is supported in the haplodiploid spider mite we studied here. According to the estimated minimum genetic distance for total reproductive isolation in parent population crosses in this study and previous work, a genetic distance of 0.15–0.21 in mtDNA (COI) appears required for speciation in spider mites. Variations and asymmetries in the degree of reproductive isolation highlight the importance of reinforcement of prezygotic reproductive isolation through incompatibility and the importance of cytonuclear interactions for reproductive isolation in haplodiploid spider mites. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s12862-021-01896-5

Sulfatide species with various fatty acid chains in oligodendrocytes at different developmental stages determined by imaging mass spectrometry

HSO3‐3‐galactosylceramide (Sulfatide) species comprise the major glycosphingolipid components of oligodendrocytes and myelin and play functional roles in the regulation of oligodendrocyte maturation and myelin formation. Although various sulfatide species contain different fatty acids, it is unclear how these sulfatide species affect oligodendrogenesis and myelination. The O4 monoclonal antibody reaction with sulfatide has been widely used as a useful marker for oligodendrocytes and myelin. However, sulfatide synthesis during the pro‐oligodendroblast stage, where differentiation into the oligodendrocyte lineage has already occurred, has not been examined. Notably, this stage comprises O4‐positive cells. In this study, we identified a sulfatide species from the pro‐oligodendroblast‐to‐myelination stage by imaging mass spectrometry. The results demonstrated that short‐chain sulfatides with 16 carbon non‐hydroxylated fatty acids (C16) and 18 carbon non‐hydroxylated fatty acids (C18) or 18 carbon hydroxylated fatty acids (C18‐OH) existed in restricted regions of the early embryonic spinal cord, where pro‐oligodendroblasts initially appear, and co‐localized with Olig2‐positive pro‐oligodendroblasts. C18 and C18‐OH sulfatides also existed in isolated pro‐oligodendroblasts. C22‐OH sulfatide became predominant later in oligodendrocyte development and the longer C24 sulfatide was predominant in the adult brain. Additionally, the presence of each sulfatide species in a different area of the adult brain was demonstrated by imaging mass spectrometry at an increased lateral resolution. These findings indicated that O4 recognized sulfatides with short‐chain fatty acids in pro‐oligodendroblasts. Moreover, the fatty acid chain of the sulfatide became longer as the oligodendrocyte matured. Therefore, individual sulfatide species may have unique roles in oligodendrocyte maturation and myelination

Additional file 2 of Patterns of reproductive isolation in a haplodiploid mite, Amphitetranychus viennensis: prezygotic isolation, hybrid inviability and hybrid sterility

Additional file 2: Table S2. Quasibinomial generalized linear models used in the lines in each female population in Fig. 4a

Additional file 1 of Patterns of reproductive isolation in a haplodiploid mite, Amphitetranychus viennensis: prezygotic isolation, hybrid inviability and hybrid sterility

Additional file 1: Table S1. Number of eggs laid during the first five days of the oviposition period, hatchability, survival rate of immature stages, female ratio of offspring and female offspring per ovipositing female in crosses

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

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 byproduct 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, analyzing 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 postmating pre- and postzygotic 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 (mtDNA and nDNA) 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 postmating, prezygotic component contributes most to reproductive isolation. As expected, the strength of postmating 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 behavioral relationships between the forms. Our study advocates the versatility of haplodiploid animals for investigating the evolution of reproductive barriers