Comparative Embryology

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.

Sex Determination

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.

Mating System

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?
The nematode vulva is an ideal system to study changes in cell signaling. The nematode vulva is a complex structure through which eggs are laid; it connects the uterus to the outside environment and is an essential component of the nematode body plan. As such, it is presumably a homologous structure among all nematodes. In C. elegans, the vulva is composed of the descendents from three epidermal cells. The principal cell interactions that coordinate vulval development in C. elegans involve only these three cells and an organizing cell of the gonadal primordium, i.e. four cells in total. The development of the vulva in many other nematodes also involves a small number of homologous cells. Yet despite the homology of the vulva and the cells involved among nematode species, a large number of changes have been noted in the signaling that occurs between these cells to regulate development of the adult structure.
Lastly, like the vulva, the gonad is a highly conserved complex organ in nematodes. Also like the vulva, in all nematodes its post embryonic development involves the descendants of a relative few number of cells, often two somatic precursor cells and one or two germline precursor cells. While the adult nematode gonad consists of tubular ovotesties patterned along their length, among disparate species the number of ovotesties (one or two) and the cellular composition and morphology of both germline and somatic tissues can vary greatly. Thus the nematode gonad offers insights into the evolution of all the processes required to form a functional organ such as: changes in the regulation of cell divisions to produce the proper number of cells, changes in cell signaling among both somatic and germline tissues, changes in basic cellular architecture, changes in fundamental cellular processes such as migration and cell death, and changes in the patterning of the germ line and the generation of gametes. The processes are fundamental to the development of all metazoans and changes in these processes are requirements for the evolution of novel morphologies.