Investigating the genetic load of an emblematic invasive species: the case of the invasive harlequin ladybird Harmonia axyridis

Abstract

Introduction events can lead to admixture between genetically differentiated populations and bottlenecks in population size. These processes can alter the adaptive potential of invasive species by shaping genetic variation, but more importantly, they can also directly affect mean population fitness either increasing it or decreasing it. Which outcome is observed depends on the structure of the genetic load of the species. The ladybird Harmonia axyridis is a good example of invasive species where introduced populations have gone through admixture and bottleneck events. We used laboratory experiments to manipulate the relatedness among H. axyridis parental individuals to assess the possibility for heterosis or outbreeding depression in F1 generation offspring for two traits related to fitness (lifetime performance and generation time). We found that inter-populations crosses had no major impact on the lifetime performance of the offspring produced by individuals from either native or invasive populations. Significant outbreeding depression was observed only for crosses between native populations for generation time. The absence of observed heterosis is indicative of a low occurrence of fixed deleterious mutations within both the native and invasive populations of H. axyridis. The observed deterioration of fitness in native inter-population crosses most likely results from genetic incompatibilities between native genomic backgrounds. We discuss the implications of these results for the structure of genetic load in H. axyridis in the light of the available information regarding the introduction history of this species.

Keywords: admixture; bottleneck; genetic load; heterosis; inbreeding depression; outbreeding depression.

Figure 1

A male climbing on top of the female during the mating process in the harlequin ladybird (photo courtesy of B. Serrate).

Figure 2

Mean values of lifetime performance (A) and generation time (B) of native (black squares) and invasive (white diamonds) populations for the three studied types of crosses. Note: Intra-pop = mating with an unrelated male of the same population. Inter-pop = mating with a male from a different randomly chosen population of the same biogeographical status (i.e., from an invasive or native population) and from a different status. Mean values are ± 1.96 standard error. The y axis shows low values of generation time, which correspond to high fitness, at the top, and high values of generation time (low fitness) at the bottom.