RepoWorm

Baird Lab Index

Reproductive Isolation in Caenorhabditis

Reproductive Isolation refers to all genetic mechanisms that prevent or limit gene flow between populations. A goal of the Baird Lab is to identify and characterize genes involved in the impelmentation of reproductive isolating mechanisms in the nematode genus, Caenorhabditis

In Caenorhabditis, four species are readily available experimental studies of reproductive isolation. These are:

C. elegans
C. briggsae
C. remanei, and
C. sp. strain CB5161

All of these species will mate with each other, and in five of twelve interspecific combinations, hybrid progeny are obtained. For the most part, these hybrids arrest during embryongenesis. In two combinations, some larvae and adults are obtained.

Cross-fertile interspecific combinations (males x females*)

C. elegans x CB5161 - all hybrids arrest during embryogenesis, 70% fail to gastrulate.
C. remanei x CB5161 - all hybrids arrest during embryogenesis, 45% fail to gastrulate.
C. briggsae x CB5161 - most hybrids arrest during embryogenesis, 33% fail to gastrulate, 1% arrest as L1 larvae.
C. remanei x C. briggsae - all hybrids arrest during embryogenesis.
C. briggsae x C. remanei - most hybrids arrest during embryogenesis, 5% hatch and either arrest at various larval stages or, rarely, reach adulthood. Adults invariably are female/hermaphrodites. All have defective gonads. Rare adults are produce sperm and ova and are self-fertile. All F2 hybrids arrest during embryogenesis, usually prior to first cleavage.

* C. briggsae females = sperm-depleted hermaphrodites.

Haldane's Rule in Caenorhabditis

'When in the F1 offspring of two different animal races one sex is absent, rare, or sterile, that sex is the heterozygous [heterogametic] sex.' (Haldane, 1922).

In crosses of C. briggsae males to C. remanei females, some F1 hybrids survive embryogeneis and either arrest during larval development or reach adulthood. Invariably, adult and larval hybrids are XX females/hermaphrodites. Presumably, all XO male hybrids arrest during embryogenes. Thus, in agreement with Haldane's Rule, XX homogametic females are more viable than XO heterogametic males.

An F1 C. briggsae/C. remanei adult XX hermphrodite hybrid. Note the arrested F2 embryo in the enlarged inset.

Defective Gastrulation as a Reproductive Isolating Mechanism

F1 Hybrids from crosses of C. elegans, C. briggsae, and C. remanei males to CB5161 females frequently fail to gastrulate and consequently arrest during early embryogenesis. Thus, defective gastrulation as a reproductive isolating mechanism either has arisen multiple times during the evolution of Caenorhabditis species or arose during the split of CB5161 from a common ancestor of C. elegans, C. briggsae, and C. remanei. The later model is possible as the most recent molecular phylogeny of this group places CB5161 as the most distant relative among these species (Baldwin et al., 1997). However, this phylogeny is not completely supported and interpretations based on it should be taken with caution.

Arrested F1 C. elegans/CB5161 hybrid embryos. A-B) Gastrulation-defective hybrids. C-D) Post-gastrula hybrids.