Worm Breeder's Gazette 13(4): 34 (October 1, 1994)

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.

Evolution of Vulva Formation: Part IV: Variation in AC Position Can Cause a Shift of Vulva Formation Towards P(4-6).p.

Ralf J. Sommer, Paul W. Sternberg

HHMI & California Institute of Technology, Division of Biology 156-29, Pasadena, CA

  Vulval cell lineage analysis in Protorhabditis (kindly provided by Dr. Sudhaus)
shows that six cells P(3-8).p undergo cell divisions in the L3 stage. P(38).p have the
same cell lineage as in Caenorhabditis, but differences exist with regard to the
epidermal lineages of P3 .pand the onset of morphogenesis. P3 .pdivided in all 50
animals of Protorhabditis whose anatomy was examined, whereas it divides in 50% of
the hermaphrodites in Caenorhabditis. The third round of cell division starts with P5
.ppaand P7 .ppadividing longitudinally, followed immediately by P6 .papand P6
.ppa.The transverse division of these cells takes place before the onset of invagination
but after the AC-attachment occur. In Caenorhabditis, the corresponding cells divide
after the onset of invagination. Vulva development in Protorhabditis is induced by the
gonad: after ablation of Z(1,4) all VPCs generated 3260 lineages (3/3 animals).
  Lineage analysis in Protorhabditis revealed that in some animals the position of
the AC varies with respect to the VPCs. In the early L3 stage of most animals the AC
was located dorsal to P6 .p,as in Caenorhabditis. However, in some animals the AC
was observed between P6 .pand P5 .por dorsal to P5 .p.The exact position of the AC
seems to change within individuals; in one animal the AC changed its position from P5
.pto P6 .pwithin 2 hours. Further lineage analysis of AC-shifted worms indicate that
P(4-6).p instead of P(5-7).p can form the vulva. In these worms the vulva lineages are
normal 1260 and 2260 lineages and P5 .p,as the 1260 cell, makes a normal AC contact in the
four cell stage. In other animals hyperinduction was observed with P4 .pbeing
completely or partially induced. Anatomical observation of Protorhabditis females in
the young L4 stage revealed that 31/50 have a normal vulva formed by P(5-7).p. In
12/50 animals P4 .pdisplayed partial or complete induction, perhaps due to the AC
being located between P5 .pand P6 .pwhen induction took place. In these animals, the
contact of the AC to the 1260 cell was finally established to P6 .p.In 6/50 animals the vulva
shifts towards P(4-6).p with P(7,8).p having the 3260 cell fate. The AC positioned dorsal
to P5 .pmight cause this transformation. In one animal the vulva was shifted and in
addition, P3 .pwas partially induced. We suggest that both shifted vulvae and
hyperinduction are caused by altered positioning of the AC. However, we cannot rule
out the possibility that additional alterations in the underlying genetic program exist in
  We speculate that variability in AC position was the origin for the generation of
vulva formation by four VPCs, as occurs in Panagrellus redivivus. In Panagrellus ,
the vulva is formed by P(5-8).p. P6 .pand P7 .phave the 1260 fate and the AC is located
between P6 .pand P7 .p.One might speculate that starting from a variable AC position,
the AC could be fixed between two VPCs. Additional evolutionary changes of the
genetic program for vulva determination and differentiation might allow both cells to
adopt the 1260 fate, as it is seen in Panagrellus .