High fusibility and chimera prevalence in an invasive colonial ascidian

Maria Casso,Davide Tagliapietra,Marta Pascual,Xavier Turon

doi:10.1038/s41598-019-51950-y

Abstract

The formation of chimeric entities through colony fusion has been hypothesized to favour colonisation success and resilience in modular organisms. In particular, it can play an important role in promoting the invasiveness of introduced species. We studied prevalence of chimerism and performed fusion experiments in Mediterranean populations of the worldwide invasive colonial ascidian Didemnum vexillum. We analysed single zooids by whole genome amplification and genotyping-by-sequencing and obtained genotypic information for more than 2,000 loci per individual. In the prevalence study, we analysed nine colonies and identified that 44% of them were chimeric, composed of 2–3 different genotypes. In the fusion experiment 15 intra- and 30 intercolony pairs were assayed but one or both fragments regressed and died in ~45% of the pairs. Among those that survived for the length of the experiment (30 d), 100% isogeneic and 31% allogeneic pairs fused. Fusion was unlinked to global genetic relatedness since the genetic distance between fused or non-fused intercolony pairs did not differ significantly. We could not detect any locus directly involved in allorecognition, but we cannot preclude the existence of a histocompatibility mechanism. We conclude that chimerism occurs frequently in D. vexillum and may be an important factor to enhance genetic diversity and promote its successful expansion.

Highlights

The formation of chimeric entities through colony fusion has been hypothesized to favour colonisation success and resilience in modular organisms
In B. schlosseri there is no evidence of an improvement in growth rates, reproduction or survivorship associated to chimerism, so other ecological or evolutionary advantages should favour chimerism in this species[21]
An average of 2,510,418 raw reads per sample were obtained in the prevalence study at the Ebro Delta and 79,1% remained after the sequence quality filtering stage

Summary

Introduction

The formation of chimeric entities through colony fusion has been hypothesized to favour colonisation success and resilience in modular organisms It can play an important role in promoting the invasiveness of introduced species. Chimera formation in botryllid ascidians is the most studied system, in Botryllus schlosseri[15] In this system, a single gene locus with multiple alleles determines the outcome of the colony contacts. Increased fusion rates among different colonies could result in higher genetic diversity and richer gene expression patterns, promoting the invasiveness of the species and turning the disadvantage into an advantage for the founder population[13]. Chimerism has been suggested as a driving mechanism of the species’ remarkable invasiveness[32,33]

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