Evolution of left–right asymmetry in side-resting (pleurothetic) bivalves: a test of genes as leaders or followers

ABSTRACT Side-resting (pleurothetic) lifestyles evolved independently many times across the Bivalvia in free-lying, byssally attached and cementing lineages. Within some species individuals lie indifferently on either their right or left side (randomly pleurothetic), whereas in others they lie consistently on one side (left-pleurothetic or right-pleurothetic). Morphological differences between valves (asymmetries) typically accompany this behaviour, so it can be studied widely across living and fossil bivalves. Because direction of asymmetry is rarely inherited in cases of random asymmetry, any evolutionary transition from randomly pleurothetic to fixed (left or right) qualifies as a case of ‘genes as followers’. Origins of left-pleurothetic or right-pleurothetic taxa directly from upright (orthothetic) ancestors would represent examples of ‘genes as leaders’. I therefore surveyed pleurothetic lifestyles across the Bivalvia. By mapping pleurothetic states onto a comprehensive phylogenetic tree that included both living and fossil families, I assessed the prevalence of these alternate modes of evolution. Various lines of evidence were also used to hypothesize phylogenetic relations within the Chamidae—the only living bivalve family exhibiting all three pleurothetic states. Randomly pleurothetic forms (direction not inherited) arose from orthothetic ancestors about half as often as fixed pleurothetic forms (direction is inherited). Unlike many other animal groups, fixed pleurothetic forms rarely arose from randomly pleurothetic ancestors, suggesting a ‘genes as leaders’ mode of evolution was most prevalent. However, many fascinating examples of randomly pleurothetic forms among Upper Ordovician and Silurian bivalves suggest that evolutionary transitions from random to fixed may have been more common. Also, randomly pleurothetic forms were short-lived in some fossil lineages, which would reduce the likelihood of them being detected. Hopefully, this preliminary analysis will motivate further study of (1) phylogenetic relations of key groups like the Chamidae, and basal Limida, Ostreida and Pterioidea, and (2) morphological variation among early members of other pleurothetic bivalve groups.