Zhang, Liangyu et al. (2017) International Worm Meeting "A compartmentalized signaling network mediates crossover assurance and crossover interference in meiosis."

Kohler, Simone, Bohn, Regina, Dernburg, Abby, Zhang, Liangyu

[International Worm Meeting, 2017]

Meiotic crossover control is a central mystery of sexual reproduction. During meiosis, crossover between homologous chromosomes is tightly regulated to ensure proper segregation. Each chromosome pair typically undergoes at least one crossover (crossover assurance) but these exchanges are also strictly limited in number and widely spaced along chromosomes (crossover interference). This has implied the existence of chromosome-wide signals that regulate crossovers, but their molecular basis remains mysterious. Early this year, we reported that the synaptonemal complex, a polymer that assembles between homologous chromosomes during meiosis, forms a phase-separated, liquid crystalline compartment. We suggested that this could explain how the synaptonemal complex might act as a conduit for signals that mediate crossover control along paired chromosomes. We recently characterized a family of four RING finger E3 ligases in C. elegans. We found that these proteins act as two heterodimers within the synaptonemal complex, creating a self-extinguishing circuit that controls crossover designation and maturation. These proteins also act at the top of a hierarchical chromosome remodeling process that enables crossovers to direct stepwise segregation. Work in the other organisms indicates that analogous mechanisms mediate crossover control in most eukaryotes.

Ana S Cunha et al. (1999) International C. elegans Meeting "Not all nematodes have constant cell lineages"

Ana S Cunha, Armand M Leroi, Ricardo B R Azevedo

[International C. elegans Meeting, 1999]

Nematodes are widely thought to be 'eutelic', that is, each adult tissue is made up of an invariant number of cells. We wondered whether this was always true, and so counted the numbers of cells in the hypodermal syncytium (hyp-7) in 15 different free-living species (Rhabditids, Panagrolaimids, and Cephalobids). Among species, mean hypodermal cell number varies 3-fold, but cellular constancy (as measured by among-individual variance in cell number) varies by 3 orders of magnitude. Furthermore, there is a strong positive correlation between the mean and variance in cell number. Thus, Panagrellus redivivus has approximately 3x as many hypodermal cells as C. elegans , but is 300x as variable. We reasoned that there are two basic ways in which one species might come to be more variable than another. First, it could be some species are more variable just because they have more complex lineages. Second, it could be that some species are more variable because they have a higher error rate during particular cellular events. To test among these alternatives, we first determined the canonical V cell lineages of two species, Oscheius myriophila and Rhabditella octopleura . Then we modelled these lineages (and those of C. elegans , and P. redivivus ) computationally, introducing plausible kinds of variation in cell divisions and fates into the simulation. These simulations generate predictions, for a given canonical lineage, of the relationship between cellular error rates and variance in adult cell number. We found that the enormous variance observed in P. redivivus (and in some other species such as Rhabditoides regina ) can only be explained by far higher error rates in V cell lineages than are found in C. elegans . Notably, Sternberg and Horvitz, when lineaging P. redivivus , observed variant V cell lineages of a sort that have never been found in C. elegans .

Karin Kiontke et al. (2007) International Worm Meeting "Evolution of male ray patterns in rhabditid nematodes."

Rocio Ng, Karin Kiontke, David H. A. Fitch, Alena Kolychkina

[International Worm Meeting, 2007]

One of the most important diagnostic character for nematodes related to C. elegans is the pattern of sensory organs—rays (genital papillae) and phasmids—in the male tail. This pattern is highly conserved within a species, but it varies between species and groups of species. Our objective is to establish the ray pattern in the stem species of rhabditids and to reconstruct the evolutionary changes which occurred within this taxon. Specifically, we look at the number of rays, the arrangement of homologous rays, and the position of phasmids relative to rays in adults and during development using DIC microscopy, MH27 antibody staining of adherens junctions, and cell ablations. Projecting the character differences onto a phylogenetic tree for rhabditids, we determine the number and kinds of evolutionary changes. We find that the number of rays was fixed early in rhabditid evolution to 9 pairs, just as the number of digits was fixed early in vertebrate evolution to 5. Later, several losses of rays but no gain occurred. Our methods allow us to homologize individual rays in the different species and thus to reconstruct the changes in their position. A significant change is the pronounced posterior and dorsad displacement of the first ray in diplogastrids. The position of the phasmids was posterior of all rays in the rhabditid stem species. Several changes to a more anterior and more dorsal position occurred within rhabditids. The change is in every case due to a migration of rays and/or phasmids or their precursors during development. The migration can happen early in L1 or L2 (as in diplogastrids), or very late during ray morphogenesis in L4 (as in Brevibucca saprophaga). Some ray pattern characters support relationships which are otherwise only supported by molecular data. Most importantly, an anterior position of the phasmids supports a relationship of Rhabditoides inermiformis and R. regina with Pleiorhabditis.