Evolutionary significance of sexual and asexual modes of propagation in neogene species of the bryozoan metrarabdotos in tropical America

Three new Miocene-Pliocene species of the cheilostome bryozoan Metrarabdoto s from Venezuela are atypical in showing significant evidence that as many as half the colonies originated asexually (clonally) by "regeneration" from previously existing colonies, rather than almost exclusively from ancestru lar zooids (products of metamorphosis of sexually produced larvae), as is c haracteristic of the genus. The extremely low proportion of zooids (less th an two percent) recognizably committed to producing larvae (ovicelled) in t hese Venezuelan species agrees with that reported in a variety of Danian (P aleocene) genera in which clonal propagation has been reported to predomina te. However, all but two of 17 other living and fossil species of Metrarabd otos also have fewer than two percent of their zooids ovicelled, even thoug h all but one of more than 250 colony bases examined originated from ancest rulae. The lack of significant correlation in Metrarabdotos between frequen cies of ovicelled zooids and of ancestrular colonies suggests that clonal p ropagation may not have diverted resources from sexual reproduction. This i nference is supported by the retention in these species of a level of herit able morphologic variation (estimated by partitioning among-colonies and wi thin-colonies variance in zooid characters) that is commensurate with that estimated for species of Metrarabdotos in which propagation was apparently entirely by sexual means. Thus, sexual reproduction throughout the genus wa s apparently sufficient to maintain the genetic diversity from which specia tion could proceed at normal rates. As estimated by both cladistic and near est-neighbor morphologic-stratigraphic methods, the three Venezuelan specie s occupy quite different positions in the inferred phylogeny of Metrarabdot os. Thus, the elevated level of clonal propagation in these species appears to be a response to local conditions, most probably high productivity asso ciated with upwelling, that promoted more rapid vegetative growth while lea ving the level of sexual reproduction unchanged.