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Two closely related species of haplodiploid spider mites interact on mountains

Speciation is an important research area, as it increases biodiversity on Earth. When closely related species with incomplete reproductive isolation expand their distribution after some degree of differentiation and form secondary contact zones, the status of hybridization and genetic introgression provides important information for understanding speciation.

To date, speciation studies have primarily focused on diploid organisms, but haplodiploid organisms have not been investigated in depth.

In a , published in Heredity, researchers focused on two species of related haplodiploid spider mites found at different elevations in Japan and investigated their secondary contact zones on Mt. Amagi (Shizuoka Prefecture) as well as hybridization and genetic introgression in that area.

Analyses of male morphology, sex ratio, single nucleotide polymorphism using multiplexed Inter-Simple Sequence Repeat (ISSR) genotyping by sequencing (MIG-seq), and crossing experiments confirmed the extensively overlapping distribution of these two species on the mountainside.

Taken together with previous studies, the results suggest widespread secondary contact zones of the two species in the mountainside from the Shizuoka Prefecture to Kyushu.

A few hybrids were obtained from crossing experiments in the laboratory, but no obvious hybrids were found in the fields. Nonetheless, the sex ratio in the field suggested interspecific mating in the secondary contact zone. In addition, genetic introgression was detected, although very low. Moreover, a male with abnormal morphology was found, but with an unclear association with hybridization.

This study is the first to reveal hybridization and genetic introgression in spider mites in a secondary contact zone. Further research on speciation of haplodiploid spider mites is needed for complete understanding of their speciation.