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Determining the causes and evolution of reproductive barriers to gene flow between populations, speciation, is the key to understanding the origin of diversity in nature. Many species manifest hybrid breakdown when they intercross, characterized by increasingly exacerbated problems in later generations of hybrids. Recently, Caenorhabditis nematodes have emerged as a genetic model for studying speciation, and here we investigate the nature and causes of hybrid breakdown between Caenorhabditis remanei and C. latens. We quantify partial F1 hybrid inviability and extensive F2 hybrid inviability; the ~75% F2 embryonic arrest occurs primarily during gastrulation or embryonic elongation. Moreover, F1 hybrid males exhibit Haldane's rule asymmetrically for both sterility and inviability, being strongest when C. remanei serves as maternal parent. We show that the mechanism by which sterile hybrid males are incapable of transferring sperm or a copulatory plug involves defective gonad morphogenesis, which we hypothesize results from linker cell defects in migration and/or cell death during development. This first documented case of partial hybrid male sterility in Caenorhabditis follows expectations of Darwin's corollary to Haldane's rule for asymmetric male fitness, providing a powerful foundation for molecular dissection of intrinsic reproductive barriers and divergence of genetic pathways controlling organ morphogenesis.

Though much widespread in distribution, the nematode worm Caenorhabditis elegans exhibits low levels of genetic variation at the DNA sequence level, a paradox which may be partly explained by its typical self-fertilization mode of reproduction. However recent work on the genetic structure of natural populations of C. elegans from France, Scotland and Portugal suggests a substantial level of outcrossing together with finding of a very strong local population structure. To find out whether this same occurrence extrapolates to other wild isolates of worm populations, the present study was carried on worm populations from California. Here, genetic variation is studied by detecting single nucleotide polymorphisms in a random genome-wide manner using Amplified Fragment Length Polymorphism analysis (AFLP). The worms used in this study are natural isolates of C. elegans collected from parks and gardens around the Los Angeles area in southern California. Some populations sampled were a few meters apart, enabling the assessment of variation and population structure on a very local scale. As previous studies, a low overall genetic diversity was found with these worm populations. The finding of a strong population structure and high linkage disequilibrium both within and between chromosomes was also concurrent with the findings from other wild isolates. All these findings elucidate the extra-ordinary evolutionary dynamics of these sets of worm from the wild.