Multigenerational life history traits of the copepod Tigriopus from the Northwest Pacific rim in crossbreeding experiments

We screened life-history variables (survival, first batch hatchability, metamorphosis rate, sex ratio) during several generations following crossbreedings of Tigriopus japonicus (3 localities in Korea, Japan, and Hong Kong) and Tigriopus thailandensis (1 population from 1 locality in Thailand). Crossbreeding results showed incomplete reproductive separation of populations of T. japonicus and T. thailandensis. Life history variables obtained during the present study indicated both higher performance and lower survival and fecundity—depending on populations—as well as faster and slower metamorphosis rates of pure lines compared to hybrid lines. These contrasting effects were particularly evident in the crossings of geographically more distant populations, indicating that their genetic compatibility was changed by showing both hybrid vigor and hybrid breakdown effects. In the present study, the fitness of hybrids was equal to or greater than that of the control lines in the F1 generation and decreased from the F2 generation with the exception of that of the TJJ × TT hybrids. The hybrid fitness changes did not correlate with genetic divergence of the copepods. Although TJJ × TT crosses were successfully mated and brought forth offspring in the present study, their first hybrid generations did not grow to adults and died. Survival percentage dropped drastically for both mating combinations, TJH-M × TT-F (P < 0.01) and TJH-F × TT-M (P < 0.05) and no 3rd generation (F3) was produced for this mating pair combination. Metamorphosis rate was similar to the above survival percentage. Sex ratios of TJH-M × TT-F in F2 generation were significantly lower than that of the control lines (P < 0.05). No successful breeding of hybrid lines in the F3 generation was indicated since the hybrids were not grown to copepodid stages. Fecundity, survival, metamorphosis rate, sex ratio and number of successful breeding lines of the hybrid and the control lines for T. japonicus Japan strain (TJJ) and T. thailandensis showed little changes in successive generations. In the crossbreeding of TJJ and TT, the fecundities of both females (F0) which were mated with different species were significantly higher than that of the control line (P < 0.05). Although F1 hybrids (TJJ-F × TT-M) showed similar survivorship and metamorphosis rate to control lines, none of the surviving copepods reached the adult stage during a 16‐day period contrary to the control lines. Similarity scores and genetic distance between three T. japonicus populations and T. thailandensis using partial mitochondrial COI and complete ITS1, -2 rDNA sequences revealed large genetic differences. Tigriopus from Hong Kong showed the highest DNA similarity (in terms of COI and ITS) and lowest genetic distance to T. thailandensis. Tigriopus from Japan and from Korea both showed a lower DNA similarity and higher genetic distance compared to T. thailandensis. Comparison of CO1 amino acid (aa) sequence from different isolates shared the highest similarity between T. japonicus from Japan with T. japonicus from Hong Kong with 97.0 and the lowest score was obtained from the geographically most distant populations, T. japonicus from Busan and T. thailandensis from Bangkok with 87.0. Comparison of CO1 and ITS protein and DNA similarity (%) and calculated genetic distance showed that T. thailandensis had a protein similarity of 100% with T. japonicus from Hong Kong, and also a highest DNA similarity of COI and ITS (of 94.6% and 98.4%, respectively). This correlated with the lowest genetic distance of 0.05% and 0.006% for COI and ITS, respectively, of this geographically closest Tigriopus populations. The overall analysis showed that T. japonicus populations from Japan and Korea were comparatively similar but genetically distant to T. thailandensis. The present crossbreeding experiment demonstrated that 2 populations of Tigriopus that were separated as the 2 species, T. japonicus and T. thailandensis solely on morphological grounds belonged in fact to one biological species.

► No complete reproductive separation of T. japonicus and T. thailandensis.
► Distinction of 4 Tigriopus groups of presumably subspecies status.
► Both higher and lower fitness (survival and fecundity) depending on populations.