Dimorphic development in Streblospio benedicti: genetic analysis of morphological differences between larval types.

The marine polychaete Streblospio benedicti exhibits two distinct larval types, making it a model for the study of developmental evolution. Females produce either large eggs or small ones, which develop into distinct lecithotrophic or planktotrophic larvae with concomitant morphological and life-history differences. Here, we investigate the inheritance of key morphological traits that distinguish the larval types. We used genetic crosses to establish the influence of maternal and zygotic differences on larval phenotypes. We found a large maternal effect on larval size and the number of larval chaetae, while the number and length of these chaetae were also strongly influenced by zygotic genotype. Interestingly, the distribution of larval phenotypes produced by these crosses suggests traits intermediate to the two parental types should not be uncommon. Yet, despite gene flow between the types in natural populations, such intermediates are rarely found in nature, suggesting that selection may be maintaining distinct larval modes.