Five new Caenorhabditis species from Indonesia provide exceptions to Haldane's rule and partial fertility of interspecific hybrids.

Given the interest in the biogeography and diversity of the Caenorhabditis genus, we established a collection of these nematodes from field surveys on 4 Indonesian islands. We isolated over 60 Caenorhabditis strains belonging to 10 species. Five species were previously known from other locations: Caenorhabditis briggsae, which was predominant, Caenorhabditis tropicalis, Caenorhabditis nigoni, Caenorhabditis brenneri, and Caenorhabditis elegans. The 5 other species are new discoveries for science, and we describe them here as Caenorhabditis indonesiana, Caenorhabditis malinoi, Caenorhabditis ceno, Caenorhabditis brawijaya, and Caenorhabditis ubi. RNA sequence analysis of 1,861 orthologous genes placed all species from Indonesia in the Elegans group of Caenorhabditis species. Four of the new species belong to a Sinica subclade of species so far only found in an East Asia-Indo-Pacific world region. The fifth new species, C. indonesiana, appears as the sister of the C. tropicalis-Caenorhabditis wallacei pair, both also found in Indonesia. The present findings are thus consistent with diversification in the Elegans group having occurred in this world region. Crosses between closely related species showed counterexamples to Haldane's "rule": for several pairs of species, in one cross direction, we only found hybrid males. In addition, we found a pair of species that could partially interbreed: C. ubi (East Java) with C. sp. 41 (Solomon islands), with the hybrid males in one cross direction being fertile. Such closely related species pairs are good models for genetic studies of incompatibilities arising during speciation.