Fitness trade-offs and developmental constraints in the evolution of soma: an experimental study in a volvocine alga.

Abstract

Background: The evolution of mortal somatic cells was a critical step in the evolution of complex body plans and the major radiations of multicellular life. In the volvocine green algae, somatic cells are hypothesized to mitigate an increasing cost of reproduction as colony size increases, primarily by providing motility to the colony during reproduction.

Questions: Does selection on colony size cause an evolutionary response in proportion of somatic cells? Does the effect of selection on colony size differ in environments that differ in the importance of motility?

Methods: We subjected an outcrossed population of the volvocine alga Pleodorina starrii to selection on colony size in still and mixed environments. After approximately 40 generations with periodic selection, we estimated the relationship between colony size and proportion of soma in evolved colonies from both environments.

Results: In the largest size category, colonies selected in the still environment (in which motility is hypothesized to be more important) had a higher proportion of soma than those from the mixed environment. Within-strain variation in cell number was surprisingly large: up to 16-fold for some genotypes. The positive among-species relationship between colony size and proportion of soma was paralleled within the larger (16- to 64-celled) colonies of P. starrii, but not within the smaller (4- and 8-celled) colonies, which had the highest proportions of soma, suggesting the existence of an evolutionary constraint preventing optimization of soma in the smallest size classes.