[6] Nevertheless, hybrids between these two species are subject to Haldane's law: heterogametic offspring (males) are much less viable than females.

[7] Hybrid male inviability primarily manifests itself during embryonic development and is most pronounced at lower growth temperature.

This sterility is at least partially caused by the presence of either of two X-chromosomal subsequences from C. briggsae, either of which is associated with abnormal transcriptional downregulation of C. nigoni autosomal genes encoding spermatogenic functions; this downregulation may be due to abnormal upregulation in hybrids of a subset of 22G RNAs specifically targeting the down-regulated spermatogenic genes.

[12] In parallel work using an independently produced third-generation genome assembly of C. nigoni, Ren et al. (2018) analyzed whole-genome alignments of the chromosomes of C. nigoni to C. briggsae;[13] they report that the two genomes have broad chromosomal synteny, but also have many intra- and inter-chromosomal sequence rearrangements.

These rearrangements are likely to impede meiotic recombination between chromosomes of the two species, and might also cause partial inviability and infertility of interspecies hybrids.