It has long been hypothesized that small changes in the pattern of cell division, cell migration, and programmed cell death in early embryogenesis could result in significant patterning differences latter in development due to altered cell-cell signaling. In turn these altered developmental programs could lead to large changes in tissue and body morphologies. This field of comparative embryology examines the early growth of related animals to try to identify these types of differences and to correlate them with later developmental differences. As nematodes have largely invariant cell lineages from a single cell zygote to the adult and as the complete lineage is known for C. elegans. Nematodes are ideal for this sort of comparative approach. Indeed studies have shown that specific changes in the pattern of blastomeres divisions underlie some of the major divisions of the nematode phylum.
As has been already stated the genetic pathways that control sex determination change very rapidly. One hypothesis has suggested that the down stream activators that directly regulate female and male specific genes are in fact highly selected and thus conserved. However, the upstream factors that determine the sex of the animal are not so subject to selection and thus are freer to change and to add additional levels of regulation. This is one focus of work on sex determination that can be easily studied in closely related nematodes by using forward and reverse genetic approaches to identify sex determination genes in related animals to study the actual conservation of individual components both at the sequence level and the pathway level.
A second rapidly evolving trait under study in nematodes is mating
systems. Throughout the nematode phyla, as for C. elegans,
many nematodes have a hermaphroditic sex (essentially a female that
produces sperm as well as eggs) and a male sex. Often some of the
closest relatives to these species exhibit a gonochoristic mating
system, the species has a female sex and a male sex. This implies
that switching between mating systems is very rapid. The phylogenetic
relationships within some nematode families suggest that hermaphroditism
has arisen many times independently. The independent acquisition of
the same trait in discrete animal and plant lineages is a common occurrence
in nature. Are the same genes involved in each evolutionary event?
What selective pressures lead to these events? If they are the same
genes, why can’t the same solution arise in another way?