JOURNAL OF CRUSTACEAN BIOLOGY, 8(4): 618-619, 1988 EVOLUTIONARY TRANSFORMATIONS AND DOLLO'S LAW Frank D. Ferrari In a recent article, Dahl (1987) ques- tioned the transfer by Schram (1986) of Phyllocarida from the Malacostraca to the Phyllopoda. Dahl's doubts centered on Schram's proposal that several character states?antennular flagellum, mandibular palp, and pleopods?have been lost and subsequently redeveloped during the evo- lutionary history of the Phyllopoda. Dahl found such evolutionary transformations unlikely. In dismissing the argument for loss- and-redevelopment, Dahl noted there is no empirical evidence of this in the fossil re- cord and he stated (p. 725) "the introduc- tion of temporary loss and redevelopment of the same morphological characters are not readily accepted by evolutionists," and in passing referred to Dollo's Law. DoUo's Law or Rule?that "structures that had been lost in evolution can never be reacquired exactly in the same way" (Mayr, 1982: 609) or more broadly "that evolution is irre- versible and that structures and functions once lost can never be regained" (Lincoln et ai, 1982: 71)?is an intuitive construct about evolution. However, an underlying assumption of the above-mentioned defi- nitions of Dollo's Law is that all phenotypic change is equated with permanent altera- tion of the genotype. This assumption too easily confuses loss of phenotypic structure caused by loss or reorganization of geno- typic structure with loss of phenotypic structure caused by repression of genotypic function. I suggest that for evolutionary transformations among crustaceans, Dol- lo's Law should not be strictly applied nor should hypotheses about loss-and-redevel- opment of characters be excluded from con- sideration. That the determination of arthropod skel- etal architecture commonly results from repression of gene function is clear from studies of homeotic gene complexes that control development of insect body seg- ments (Lewis, 1978, 1981; Kaufman et ai. 1980; Beeman, 1987). The evolution of these gene complexes seems to have suppressed leg development on abdominal segments, and in fruit flies suppressed the posterior pair of wings, while promoting haltere for- mation, in the four-winged ancestor (Lewis, 1978). Many genes in these complexes con- tain homeobox regions whose proteins pre- sumably bind to DNA of still other genes. Activation or repression of these down- stream genes result in the specific segment morphologies. Changes in timing of gene repression, or alteration of represser mol- ecule structure or concentration conceiv- ably can restore ancestral gene function and result in development, in the case of fruit flies (Lewis, 1963), of an ancestral, bithor- ax/postbithorax, double-winged architec- ture. Phenotypic evidence of gene repression in crustaceans has been deduced from trans- formations in swimming leg segment num- bers during development of several cope- pods by Ferrari (1988). Examples include reduction of male leg 5 endopod of Heter- ocope weismanni described by Elster (1932), reduction of male leg 4 endopod of Ela- phoidella bidens coronata described by Car- ter and Bradford (1972), reduction of male leg 2 endopod of Diarthrodes cystoecus de- scribed by Fahrenbach (1962), loss of legs 1 and 2 of Acanthochondria cornuta de- scribed by Heegaard (1947), loss of legs 1 and 2 of Alella macrotrachelus described by Kawatow et al. (1980). Reductions in num- bers of ramal segments, or loss of a swim- ming leg (which occasionally is accom- panied later by leg redevelopment) during an ontogenetic sequence are inferred to re- sult from repression of a genetic system con- trolling the expression of leg morphology. It is difficult to infer the direction and limits of phylogenetic transformation by ex- trapolating from observed changes in phe- notype during ontogeny, or to determine to what extent genes presently repressed have been expressed historically. However, de- velopmental systems in which gene function and the resulting phenotype are regulated by gene repression suggest that considering 618 FERRARI: DOLLOS LAW 619 all phenotypic change irreversible is too strict an approach to evolutionary transfor- mations. ACKNOWLEDGEMENTS Carel von Vaupel Klein. Rijksmuseum van Natuur- lijke Historie. Leiden; Frederick R. Schram, San Diego Natural History Museum; Edward B. Lewis. California Institute of Technology; and Mark J. Grygier and Rich- ard O'Grady, Smithsonian Institution, encouraged de- velopment of this note and provided thoughtful re- LlTERATURE CiTED Beeman. R. 1987. A homeotic gene cluster in the red flour beetle. ?Nature 327: 247-249. Carter. M.. and J. Bradford. 1972. Postembryonic development of three species of freshwater harpac- ticoid Copepoda.?Smithsonian Contributions to Zoology 119: 1-26. Dahl. E. 1987. Malacostraca maltreated?the case of the Phyllocarida.?Journal of Crustacean Biology 7: 721-726. Elster, H.-J. 1932. Monographische Studien an//e/- ercope weismanni Imhof I. Postembryonal-Ent- wicklung und Morphologie. 2 Teil. ?Internationale Revue der gesamtcn Hydrobiologie und Hvdrogra- phie 27: 177-233. Fahrenbach, W. 1962. The biology of a harpacticoid copepod.-La Cellule 62: 303-376. Ferrari, F. 1988. Developmental patterns in numbers of ramal segments of copepod post-maxillipedal legs.?Crustaceana 54: 256-293. Heegaard. P. 1947. Contribution to the phylogeny of arthropods?Copepoda.?Spolia Zoologica Musei Hauniensis 8: 1-236. Kaufman. T., R. Lewis, and B. Wakimoto. 1980. Cy- togenctic analysis of chromosome 3 in Drosophila melanogasler: the homoeotic gene complex in poly- tenc chromosome interval 84A-B.?Genetics 94: 115-133. Kawatow, K., K. Muroga, K. Izawa. and S. Kasahara. 1980. Life cycle o( Alella macrolrachelus (Cope- poda) parasitic on cultured black sea-bream.?Jour- nal of the Faculty of Applied Science, Hiroshima University 19: 199-214. [In Japanese.] Lewis, E. B. 1963. Genes and developmental path- ways.?American Zoologist 3: 33-56. . 1978. A gene complex controlling segmen- tation in Drosophila.?Nalure 276: 565-570. . 1981. Developmental genetics of the bithor- ax complex in Drosophila. ? In: Developmental bi- ology using purified genes. ICN-UCLA Symposia on developmental biology using purified genes. Key- stone. Colorado. Pp. 189-208. Academic Press. New York. Lincoln, R., G. Boxhall, and P. Clark. 1982. A dic- tionary of ecology, evolution and systematics.? Cambridge University Press, Cambridge, London, New York. Pp. i-vii, 1-298. Mayr, E. 1982. The growth of biological thought.? Belknap Press of Harvard University Press. Cam- bridge. Pp. i-ix, 1-974. Schram, F. R. 1986. Crustacea.?Oxford University Press, New York, Oxford. Pp. i-xiv, 1-606. RECEIVED: 16 February 1988. ACCEPTED: 14 April 1988. Address: Smithsonian Oceanographic Sorting Cen- ter, National Museum of Natural History, Smithsonian Institution, Washington, D.C. 20560.