Geographic and reproductive isolation in the marine Harpacticoid Copepod Tisbe

Abstract

Within the genus Tisbe (Copepoda: Harpacticoida), there is a wide range of reproductive isolation. In the European area, in addition to species where (between geographic populations) the reproductive barrier is practically absent, there are others which show a varying degree of incompatibility. The present paper deals with the results of cross-breeding experiments carried out between American and European populations. The respective intrapopulation crosses served as controls. Transatlantic crosses have been attempted with the following species: T. lagunaris, T. bulbisetosa, T. holothuriae, T. battagliai, and T. clodiensis. The American and European populations of the first 4 species are interfertile, and the F₁ hybrids produce viable offspring. Moreover, the interpopulation crosses, compared to the controls, exhibit a slight superiority whose nature is, however, still unclear. The fifth species, T. clodiensis, behaves differently. The absence of viable offspring from interpopulation crosses, or the death of the hybrids before reaching the adult stage, indicate that, in this species, the barrier to gene flow is complete. Populations of T. clodiensis show, in addition, signs of “relative intraspecific incompatibility”. The problem of the different degree of reproductive isolation in the genus Tisbe, with special regard to the role played by ecological factors in determining the isolation patterns, is discussed.