טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentKolotuev Irina
SubjectEvolution of Vulva Formation and Cell Fusion in Nematodes
Related to Caenorhabditis elegans
DepartmentDepartment of Biology
Supervisor Professor Benjamin Podbilewicz
Full Thesis textFull thesis text - English Version


Abstract

Evolution of organogenesis may be understood by comparing organ formation and identification of specific changes in cellular events. Vulva formation in various nematode species (C. elegans in particular) is a well studied paradigm of organ formation and evolution. Here I focused on two major projects: 1) The evolution of vulva formation in the Rhabditida family 2) The role of the HOX gene mab-5 in vulva formation and its interactions with the fusion gene eff-1 in C. elegans. Both projects revealed conserved molecular mechanism of vulva formation between C. elegans and related nematodes.

From my previous studies of vulva morphogenesis I formulated a hypothesis that correlates between the division orientation of vulval cells and the final number of vulva composing rings. According to it, one ring is formed from each primary vulval cell and its counterpart. The two exceptions: (a) final transverse division and (b) longitudinal division of the outermost vulval cells, fuse between themselves and only then form one ring. To test this hypothesis I studied 12 species related to C. elegans and found correlation between the patterns of vulva cell divisions and the final number of vulval rings. To examine the ring hypothesis I analyzed vulva formation in lin-11 mutant of C. elegans, in which some cells that usually divide transversely, change the orientation of division axes and divide longitudinally.  The ring hypothesis predicted that an additional ring should be formed and this is exactly what I found. 

HOX genes are a group of genes that were shown to specify the anterior - posterior body axis.  In C. elegans, two of 6 HOX genes were found to be involved in vulva formation (lin-39 and mab-5). Mutations in the HOX gene mab-5 lead to aberrant male phenotypes while the hermaphrodites are egg laying competent.  In P. pacificus mab-5, hermaphrodites are egg laying defective and possess bivulva structures posterior to the real vulva.  I found that in 4% of C. elegans mab-5 different bivulvae can be obtained as in P. pacificus. To enhance the vulval phenotype of mab-5 mutants I used the background of the fusion defective mutant eff-1. Epithelial cells in eff-1 mutants remain unfused unlike in wild type. In the double mutant mab-5;eff-1, a bivulva phenotype showed synergistic effect (70%). These findings broaden the previously known function of mab-5 in hermaphrodites and show that the role of mab-5 in hermaphrodites has been conserved during evolution.