Intraspecific genetic divergence in the Baikal amphipod species Gmelinoides fasciatus does not lead to reproductive isolation

Cryptic diversity is a widespread phenomenon and poses significant challenges for modern taxonomy. A principal question is determining the threshold level of differences for species delimitation. Earlier, we proposed Lake Baikal amphipods—species flocks with varying levels of intraspecific genetic divergence—as a model group. Here, we focus on Gmelinoides fasciatus, which exhibits clear intraspecific genetic divergence (up to 8% uncorrected pairwise distances in cytochrome oxidase subunit I (COI) nucleotide sequences) that is still lower than that of reproductively isolated species complexes (Drozdova et al., 2024, doi: 10.18699/vjgb-24-36).

Animals were sampled in Baikal littoral near Bolshie Koty (western shore of the lake) and Tankhoy (eastern shore). Using published COI sequences (Gomanenko et al., 2005; doi: 10.1007/s11177-005-0179-5), we developed a quantitative PCR system for genotyping based on characteristic substitutions. The presence of a prezygotic reproductive barrier was checked with experimental mating trials as described for other species (Drozdova et al., 2025; doi: 10.3390/d17110781).

First, we checked if two studied locations were inhabited by different groups. Genotyping of 55 specimens per location confirmed this hypothesis and also showed that each population was genetically homogenous. Second, prezygotic reproductive compatibility was tested, and no significant difference from random amplexus formation was observed: in 8 out of 16 trials amplexus was formed by animals from the same population; in 6, by animals from different populations; and in 2 cases, no amplexus formed.

Thus, despite a clear distinction between genetic lineages, no prezygotic reproductive barrier was observed. Combined with data on Eulimnogammarus species, this result suggests similar divergence thresholds for reproductive incompatibility in these groups. As these genera occupy different positions in the phylogeny of the formal genus Gammarus, these results may apply more broadly to gammaroid amphipods.

The work is supported by RScF (22-14-00128-P) and the Ministry of Science and Higher Education (FZZE-2024-0008).