Artificial selection on lifespan in the appendicularian oikopleura dioica.

Abstract

Artificial selection experiments are essential tools for understanding the variation patterns and the genetic basis of adaptation traits in evolutionary processes. Model organisms constitute an attractive target for several studies due to their simplified systems and short lifecycles. Oikopleura dioica (Fol, 1872) is a species of free-swimming pelagic tunicate widely distributed over temperate and warm waters. It is semelparous, dioecious and takes 5-8 days for completing their lifecycle at 15ºC. O. dioica is important as a model species for developmental and phylogenetic studies, and the sampling and culturing methodology have been optimized. The aim of this Thesis is to test the possibility of conducting an artificial selection experiment to develop breeding lines with contrasting longevity as suggested in a previous study. To measure lifespan, two approaches were developed. One, based on the population size during reproduction (Lp) and another, based on direct timing of random individuals during reproduction (Ld). We successfully reared three consecutive generations, with an average lifespan of 6.9±1.44 (mean±SD). The effect of artificial selection resulted non-significant (p=0.873 for Lp, p=0.051 for Ld) in the entire experiment, although a significant plastic response to the population density (d0) prevailed over our selection treatments (p=1.08×10-5 for Lp, p=0.003 for Ld). The best model for Lp and Ld consisted on single straight lines fitted to the lifespan vs. d0 data (Lp = 0.0574×d0 + 4.3019; Ld = 0.0586×d0 + 4.436). We hypothesize that lifespan extension dependent on population density may allow dispersal by ocean turbulence to escape from adverse environments.