EVOLUTIONARY CHANGES OF DEVELOPMENTAL MECHANISMS IN THE ABSENCE OF CELL LINEAGE ALTERATIONS DURING VULVA FORMATION IN THE DIPLOGASTRIDAE (NEMATODA)

The origin of novelty is one of the least understood evolutionary phen omena. One approach to study evolutionary novelty comes from developme ntal biology. During developmental cell fate specification of the nema tode Pristionchus pacificus (Diplogastridae), five cell fates can be d istinguished within a group of twelve ventral epidermal cells. The dif ferentiation pattern of individual cells includes programmed cell deat h, cell fusion and vulval differentiation after induction by the gonad . A cell lineage comparison among species of seven different genera of the Diplogastridae indicates that the differentiation pattern of vent ral epidermal cells is highly conserved. Despite this morphological co nservation, cell ablation experiments indicate many independent altera tions of underlying mechanisms of cell fate specification. Cell fusion and individual cell competence change during evolution as well as the differentiation property in response to inductive signaling. These re sults suggest that developmental mechanisms, some of which are redunda ntly involved in vulval fate specification of the genetic model organi sm Caenorhabditis elegans, can evolve without concomitant morphologica l change.