Collin Rachel, Chaparro Oscar R, Winkler Federico, Véliz David
Smithsonian Tropical Research Institute, Apartado Postal 0843-03092, Balboa, Ancon, Republic of Panama.
Biol Bull. 2007 Apr;212(2):83-92. doi: 10.2307/25066586.
Evolutionary transitions between different modes of development in marine invertebrates are thought to be biased toward the loss of feeding larvae. Because the morphology of feeding larvae is complex and nonfeeding larvae or encapsulated embryos with benthic development often have simplified morphologies, it is presumed to be easier to lose a larval stage than to reacquire it. Some authors have gone so far as to suggest that feeding larvae, morphologically similar to the ancestral feeding larvae, cannot be reacquired. However, the larval structures of some groups, most notably gastropods, are often retained in the encapsulated embryos of species that hatch as benthic juveniles. Therefore the re-evolution of feeding larvae using the same structures may be possible in these groups. Here we present the first well-substantiated case for the recent re-evolution of feeding larvae within a clade of direct-developers. DNA sequence data show that Crepipatella fecunda, a species of calyptraeid gastropod with planktotrophic development, is nested within a clade of species with direct development, and that Crepipatella dilatata, a species with direct development, appears to be paraphyletic with respect to C. fecunda. Observation of the embryos of C. dilatata shows that the features necessary for larval feeding and swimming are retained in the encapsulated veligers, suggesting that heterochronic shifts in hatching time and changes in nurse-egg allotment could have resulted in the re-evolution of feeding larvae in this species.
海洋无脊椎动物不同发育模式之间的进化转变被认为倾向于失去摄食性幼虫。由于摄食性幼虫的形态复杂，而非摄食性幼虫或具有底栖发育的包被胚胎通常形态较为简单，因此推测失去幼虫阶段比重新获得它更容易。一些作者甚至认为，形态上与祖先摄食性幼虫相似的摄食性幼虫无法重新获得。然而，某些类群的幼虫结构，最显著的是腹足纲动物，在孵化时为底栖幼体的物种的包被胚胎中常常得以保留。因此，在这些类群中，利用相同结构重新进化出摄食性幼虫或许是可能的。在此，我们展示了首个有充分证据支持的直接发育者类群中近期摄食性幼虫重新进化的案例。DNA序列数据表明，具有浮游生物食性发育的Calyptraeid腹足纲动物Crepipatella fecunda嵌套在直接发育物种的一个类群中，而直接发育的物种Crepipatella dilatata相对于C. fecunda似乎是并系的。对C. dilatata胚胎的观察表明，幼虫摄食和游泳所需的特征在包被的面盘幼虫中得以保留，这表明孵化时间的异时性转变和保育卵分配的变化可能导致了该物种摄食性幼虫的重新进化。
