Complex patterns of actin molecular evolution in the red alga Stylonema alsidii (Stylonematophyceae, Rhodophyta)

Abstract

SUMMARYThe Stylonematophyceae is an early diverging red algal class that contains organisms in which sexual reproduction has not been reported. Previous studies have hypothesized a single copy of the actin gene in this class. Amplification of the actin gene in members of the Stylonematophyceae, including several isolates of Stylonema alsidii, reveals a more complex evolutionary history. The data support independent gene duplications in Goniotrichopsis reniformis and Stylonema alsidii. Three of the seven isolates of S. alsidii had three copies of actin. Analyses indicate that all copies are functional and not pseudogenes. Due to our lack of knowledge of the ploidy level in Stylonema and the asexual nature of these organisms, an ancient change in ploidy level, producing diploid or even triploid organisms, cannot be ruled out to explain two copies of actin within these organisms. Within Stylonema alsidii two of the actin copies (Groups 1 and 2) show similar levels of variation; p‐distances and the number of synonymous and non‐synonymous sites are roughly the same. Yet the changes are distributed differently. One group showed little shared variation among the samples, mutations therefore represent autoapomorphies, while variation in the other group is usually shared among samples (parsimony‐informative). Variation in the other group is usually shared among samples (parsimony‐informative). A third group of actin genes also observed in S. alsidii are highly divergent from the other two copies, yet they maintain all of the signatures of functional proteins. Our data reveal a complex evolutionary history of the actin gene in these species and highlight the lack of knowledge into the basic biology (ploidy level, life cycle characteristics) of this class.