[
Trends Genet,
1997]
Focused studies on model organisms with favorable features have been important for advancing many areas of biology. Nematodes have been a successful model for analyzing development. Can they also be used to study evolution? Paul Sternberg and his present and former colleagues are attempting to answer this question by studying variation of that well-described little structure, the nematode vulva. Their efforts have been well rewarded. Two recent publications extend a series of papers showing a surprising degree of evolutionary variability in vulval development among species. Could it be that comparison of nematode species will prove to be as powerful for penetrating the intimate mechanisms of evolutionary change as analysis of mutant nematodes has been to understanding mechanisms of development?
[
Experimental Parasitology,
1993]
Nematodes possess biologically unusual surfaces. The dominant feature is the cuticle, a collagen-rich extracellular matrix which acts as the exoskeleton and is synthesized in the outermost tissue layer of the organism, the epidermis or hypodermis. The cuticle, which may be 1 um or more in depth, has on its external face a lipid-rich epicuticle which in some species resembles a unit membrane. There may be an additional envelope, loosely attached and distal to the epicuticle, in the form of a surface coat or glycocalyx, or as a thin sheath retained by some larval nematodes from previous developmental stages. The organisation of these components is depicted in Fig. 1. Detailed studies of these structures are now becoming available in both parasitic and free-living nematodes such as Caenorhabditis elegans...
[
Cold Spring Harbor Symposia on Quantitative Biology,
1982]
The capping of cross-linked surface receptors on lymphocytes and other cells and the centripetal movement of surface-attached particles on crawling cells are examples of directed surface membrane movement. One possible mechanism for moving membrane components is that cytoskeletal proteins recognize cross-linked surface receptors and drag them through the membrane bilayer to one pole of the cell. Favoring this theory are the localization of actin and/or myosin under capped or mobile cross-linked surface molecules and the biochemical coisolation of actin with capped proteins. Another possibility is that movement results from flow of bulk membrane or membrane lipid between an assembly point at one pole of the cell to a disassembly point elsewhere on the surface. Capping on sessile cells and rearward membrane movement on crawling cells have different functions. The clustering of surface receptors that occurs during capping seems to play a role in the transmission of signals from surface-bound ligands across the membrane. As pointed out by Abercrombie, membrane movement on crawling cells may participate in locomotion. Crawling requires continuous assembly of new cell-substrate contact sites at the leading edge of the cell. Rearward membrane movement would result from this polarized membrane assembly. Thus, it is reasonable that these two types of direct membrane movement could be driven by different mechanisms, with cytoskeletal linkage operating in sessile cells and membrane flow occurring on crawling cells. This notion is supported by studies showing two distinct mechanisms for capping, one occurring spontaneously on motile cells and the other requiring ligand cross-linkage on lymphocytes. Here we examine the surface membrane movements on the amoebid spermatozoon of the free-living nematode Caenorhabditis elegans. This cell exhibits a pronounced morphological asymmetry, with the cellular organelles segregated into a hemispherical cell body (3-4 um dia). The cell forms a single persistent pseudopod, filled with granular cytoplasm, that extends about 4 um. Three features make the C. elegans spermatozoon ideal for studying the crawling movements of metazoan cells. First, differentiation of sessile, spherical spermatids into spermatozoa can be activated in vitro with the monovalent ion ionophore monensin so that the onset of motility can be controlled. Second, the asymmetry of the cell is clearly defined with the cell-body-pseuodopod junction visible both externally and internally. Third, sperm-defective mutants can be isolated, allowing cellular motility to be analyzed genetically. We now report that surface membrane movement on C. elegans spermatozoa occurs exclusively on the pseudopod. This movement comprises centripetal bulk-membrane flow and is not restricted to rearrangement of cross-linked membrane components. Furthermore, surface