Abstract
Hydrozoans typically develop colonies through asexual budding of polyps. Although colonies of Ectopleura are similar to other hydrozoans in that they consist of multiple polyps physically connected through continuous epithelia and shared gastrovascular cavity, Ectopleura larynx does not asexually bud polyps indeterminately. Instead, after an initial phase of limited budding in a young colony, E. larynx achieves its large colony size through the aggregation and fusion of sexually (nonclonally) produced polyps. The apparent chimerism within a physiologically integrated colony presents a potential source of conflict between distinct genetic lineages, which may vary in their ability to access the germline. To determine the extent to which the potential for genetic conflict exists, we characterized the types of genetic relationships between polyps within colonies, using a RAD‐Seq approach. Our results indicate that E. larynx colonies are indeed comprised of polyps that are clones and sexually reproduced siblings and offspring, consistent with their life history. In addition, we found that colonies also contain polyps that are genetically unrelated, and that estimates of genome‐wide relatedness suggests a potential for conflict within a colony. Taken together, our data suggest that there are distinct categories of relationships in colonies of E. larynx, likely achieved through a range of processes including budding, regeneration, and fusion of progeny and unrelated polyps, with the possibility for a genetic conflict resolution mechanism. Together these processes contribute to the reevolution of the ecologically important trait of coloniality in E. larynx.
Highlights
Coloniality is a life-history trait that confers great competitive advantage for space in crowded benthic environments
Our hypothetical distributions served as reference for the possible amount of genetic variation among offspring that could be produced by within-colony genetic variation alone
Our work reveals that colonies of E. larynx are genetically chimeric and contain multiple types of within-colony genetic relationships, namely clones and familial relationships and, surprisingly, fusion of unrelated polyps from the local population with a low degree of allele-sharing with the rest of the colony
Summary
Coloniality is a life-history trait that confers great competitive advantage for space in crowded benthic environments. The colonial hydrozoan Ectopleura larynx cannot bud indeterminately and instead achieves its large size by fusing sexually produced juvenile polyps to the adult colony. This process appears contrary to what is predicted by evolutionary theory—genetically chimeric colonies give rise to potential genetic conflict between colony members sharing resources, and might be opposed by natural selection. Evolution Letters published by Wiley Periodicals, Inc. on behalf of Society for the Study of Evolution (SSE) and European Society for Evolutionary Biology (ESEB) Our results provide an evaluation of genomic diversity in a system that uniquely uncouples coloniality from strict clonality, and suggests that in certain situations selection for large size takes precedence over selection for genetic homogeneity within an integrated colony, producing colonies containing a complex set of genetic relationships
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