MODIFICATIONS OF CELL FATE SPECIFICATION IN EQUAL-CLEAVING NEMERTEAN EMBRYOS - ALTERNATE PATTERNS OF SPIRALIAN DEVELOPMENT

The nemerteans belong to a phylum of coelomate worms that display a hi ghly conserved pattern of cell divisions referred to as spiral cleavag e, It has recently been shown that the fates of the four embryonic cel l quadrants in two species of nemerteans are not homologous to those i n other spiralian embryos, such as the annelids and molluscs (Henry, J . Q. and Martindale, M, Q, (1994a) Develop, Genetics 15, 64-78), Equal -cleaving molluscs utilize inductive interactions to establish quadran t-specific cell fates and embryonic symmetry properties following fift h cleavage, In order to elucidate the manner in which cell fates are e stablished in nemertean embryos, we have conducted cell isolation and deletion experiments to examine the developmental potential of the ear ly cleavage blastomeres of two equal-cleaving nemerteans, Nemertopsis bivittata and Cerebratulus lacteus, These two species display differen t modes of development: N. bivittata develops directly via a non-feedi ng larvae, while C. lacteus develops to form a feeding pilidium larva which undergoes a radical metamorphosis to give rise to the juvenile w orm, By examining the development of certain structures and cell types characteristic of quadrant-specific fates for each of these species, we have shown that isolated blastomeres of the indirect-developing nem ertean, C. lacteus, are capable of generating cell fates that are not a consequence of that cell's normal developmental program, For instanc e, dorsal blastomeres can form muscle fibers when cultured in isolatio n, In contrast, isolated blastomeres of the direct-developing species, N. bivittata do not regulate their development to the same extent, So me cell fates are specified in a precocious manner in this species, su ch as those that give rise to the eyes, Thus, these findings indicate that equal-cleaving spiralian embryos can utilize different mechanisms of cell fate and axis specification, The implications of these patter ns of nemertean development are discussed in relation to experimental work in other spiralian embryos, and a model is presented that account s for possible evolutionary changes in cell lineage and the process of cell fate specification amongst these protostome phyla.