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:
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.
C. remanei, and
C. sp. strain CB5161
Cross-fertile interspecific combinations (males x females*)
* C. briggsae females = sperm-depleted hermaphrodites.
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
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.
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
Arrested F1 C. elegans/CB5161 hybrid embryos. A-B) Gastrulation-defective
hybrids. C-D) Post-gastrula hybrids.