Evolutionary significance of sexual and asexual modes of propagation in Neogene species of the bryozoan Metrarabdotos in tropical America

Abstract

Three new Miocene-Pliocene species of the cheilostome bryozoan Metrarabdotos 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 ancestrular zooids (products of metamorphosis of sexually produced larvae), as is characteristic of the genus. The extremely low proportion of zooids (less than two percent) recognizably committed to producing larvae (ovicelled) in these Venezuelan species agrees with that reported in a variety of Danian (Paleocene) genera in which clonal propagation has been reported to predominate. However, all but two of 17 other living and fossil species of Metrarabdotos also have fewer than two percent of their zooids ovicelled, even though all but one of more than 250 colony bases examined originated from ancestrulae. The lack of significant correlation in Metrarabdotos between frequencies of ovicelled zooids and of ancestrular colonies suggests that clonal propagation may not have diverted resources from sexual reproduction. This inference is supported by the retention in these species of a level of heritable morphologic variation (estimated by partitioning among-colonies and within-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 was apparently sufficient to maintain the genetic diversity from which speciation could proceed at normal rates. As estimated by both cladistic and nearest-neighbor morphologic-stratigraphic methods, the three Venezuelan species occupy quite different positions in the inferred phylogeny of Metrarabdotos. Thus, the elevated level of clonal propagation in these species appears to be a response to local conditions, most probably high productivity associated with upwelling, that promoted more rapid vegetative growth while leaving the level of sexual reproduction unchanged.