Expression of Distal-less in molluscan eggs, embryos, and larvae

Distal-less (Dll) is best known as a transcription factor involved with "li mb patterning" in Drosophila melanogaster. Observations of both deuterostom e and protostome phyla have led to the suggestion that some aspect of this gene's function in "appendage" or proximal-distal "outgrowth" development i s conserved. Here we explore the possibility of other conserved roles opera ting earlier in development. We examine the expression of DLL protein durin g the early development of two molluscan classes, Polyplacophora (chiton) a nd Gastropoda (snail). Using an antibody approach, we find DLL expression i n the oocytes of a chiton (Mopalia muscosa) and in the pregastrulae through early veliger larvae of a marine snail (Kelletia kelletii). We observe ant ibody localization in the oocyte, nuclear expression in all cells of the pr egastrulae, and predominant expression in the ectoderm of postgastrulae and early veliger larvae. Comparison of our observations on spiralian taxa, th ought to have conservative development with previous work, primarily on deu terostomes, suggests the possibility of an ancient role(s) for DLL in early development. Possible functions appear to include maternal and zygotic inv olvement in the establishment of embryonic polarity, involvement in the pro cess of germ layer formation, and a role in the specification and/or differ entiation of ectoderm/epithelia. We note that the exploration of conserved gene function in early development may be clarified by examining taxa whose early development has putatively not been subject to dramatic evolutionary change.