Data from: A northward habitat expansion along with directional gene flow in two microscopic herbivores

Speciation is frequently accompanied by gene flow, a phenomenon that is
recognized as ubiquitous in nature. However, the role of gene flow in
speciation remains unclear, particularly in tiny and sympatric arthropods
that lack clear geographic isolations. This study aims to uncover how
sympatry, parapatry, and allopatry underlie the speciation process in two
monophagous mites, Epitrimerus sabinae and Glossilus juniperensis, which
belong to the E. sabinae complex and infest coniferous plant (Juniperus
chinensis). We conducted whole-genome resequencing of 298 mite individuals
from 56 populations, spanning nearly 20 years of field surveys, to
determine the drivers shaping species distribution patterns and to explore
the role of gene flow in speciation. Population structure analyses
revealed four genetic groups, corresponding to allopatric southern G.
juniperensis and northern E. sabinae, and sympatric central G.
juniperensis and central E. sabinae; the sympatric E. sabinae and G.
juniperensis populations were linked by sharing mixed ancestry. Our
demographic history analyses showed that these two species derived from an
ancestral lineage in southern China and diverged in the late Pleistocene,
followed by a northward expansion along with a varied degree of gene flow.
The genetic differentiation of both species was positively affected by
geographic distance and environmental factors. Our results reveal a
pattern of speciation in two mite species, initiated by genetic divergence
with gene exchange between two species (sympatry) and followed by genetic
isolation and habitat expansion (parapatry or allopatry). Our findings
underscore a one-way—northward—habitat expansion along with directional
gene flow in two herbivores speciation.