Worm Breeder's Gazette 5(2): 37
These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.
Anatomical differences between the gonads of the P. redivivus female and the C. elegans hermaphrodite suggested that these structures might differ in cell lineage. The female of P. redivivus is monodelphic (has one ovary) with a single reflexed gonadal arm. A uterus, spermatheca, oviduct, and ovary are directed anteriorly from the vulva; a post-vulval sac of unknown function is directed posteriorly from the vulva. The male gonads of P. redivivus and C. elegans are similar in shape but differ in the internal structure of the vas deferens. We have determined most of the postembryonic cell lineages of the somatic gonad (Z1, Z4) of the P. redivivus male and female. Early in the development of the P. redivivus female gonad Z4. pp dies; this cell is lineally equivalent to the posterior distal tip cell (dtc) of the C. elegans hermaphrodite. In C. elegans, the laser ablation of a dtc prevents growth of the relevant gonadal arm (J. Kimble, Newsletter, January 1980). Thus, the monodelphy of P. redivivus probably results from the programmed death of Z4.pp. P. redivivus may have evolved from a didelphic (two-ovaried) species. The cell divisions in the Z1 and Z4 lineages occur in two distinct periods--the early period extending from the late L1 through the mid- L2, and the late period from early L3 to mid-L4. Early period divisions are shown in the Figure along with those of C. elegans for comparison. The early lineages are similar in both sexes and both species, whereas later lineages differ as do the structures of the gonads. The early lineages of the C. elegans hermaphrodites and P. redivivus female are identical except for the cell death and the timing of divisions (in the female, Z4 divides substantially later than Z1; this first indication of an anterior-posterior asymmetry may be related to the death of Z4.pp and/or to the late period lineage differences between the cell labelled in the Figure as SA and PS). The obviously homologous structures are generated by lineally equivalent cells. Z4.app and Z4.pa give rise to the posterior sheath and spermatheca in C. elegans and the post-vulval sac in P. redivivus, which suggests that the sac is homologous to the sheath and spermatheca. The post-vulval sac may be an evolutionary vestige. In the C. elegans male, Z1.a migrates posteriorly to the distal end of the developing gonad. In the P. redivivus male, the apparently homologous cell is Z1.p, which is generated in the correct position to produce the ordered (cloacal-proximal to cloacal-distal) set of structures in the gonad. The only other difference between the two male early lineages is an extra division of Z1.p and Z4.p in P. redivivus. The linker cell (lc) and the anchor cell (ac), which appear to be homologous, define the proximal end of the gonad in the male and female/ hermaphrodite respectively. The dtc defines the distal end of the gonad. Given these homologies, the early lineage differences between the P. redivivus male and female appear to involve a rotation of Z1, whereas the differences between the C. elegans male and hermaphrodite appear to involve a rotation of Z1.p. Homologies between the structures of the two sexes remain elusive. [See Figure 1] Comparison of the early lineages of the somatic gonad in both sexes of C. elegans and P. redivivus. The lineages up to the time of the L2 molt are shown schematically. C. elegans lineages have been adapted from Kimble and Hirsh (Develop. Biol. 70: 396, 1979). The lineage tree and directions of mitosis are as defined in Sulston and Horvitz (Develop. Biol. 56: 110, 1977); anterior is drawn to the left and posterior is drawn to the right except where otherwise indicated. We do not know at present whether or not P. redivivus exhibits variability in cell fates, for example as in the case of Z1. ppp and Z4.aaa in the C. elegans hermaphrodite. Terminal cell fates are noted in lower case. Cells that divide further are indicated in capitals according to the structures they predominantly generate; a particular structure may be derived from different lineages. Alternate fates are as shown. Cell death is indicated by an 'X'. A dashed line indicates that a migration occurs. Abbreviations: dtc, distal tip cell; ac, anchor cell; lc, linker cell; sh, sheath-like cell equispaced around the developing testis (Z1.pp in P. redivivus might become a dtc in some animals, or might have dtc function although it does not reside at the distal tip); SA, anterior sheath and spermatheca (includes the oviduct in P. redivivus); DU, dorsal uterus; VU, ventral uterus; SP, posterior sheath and spermatheca; PS, post-vulval sac; VD, vas deferens; SV, seminal vesicle; ED, ejaculatory duct (proximal portion of vas deferens); DS, vas deferens and seminal vesicle.