# Microsatellite Data Indicate an Extreme Founder Event with a Single Female Lineage in the Parasitoid Wasp Monodontomerus obscurus

## TL;DR

A parasitoid wasp in Japan likely started from a single female, showing how extreme founder events can lead to population success.

## Contribution

Provides rare empirical evidence of a population founded by a single female in a natural setting.

## Key findings

- The wasp population in Japan was likely founded by a single female, as shown by extremely low genetic diversity.
- Post-invasion mutations generated new genotypes in the population.
- The species' mating system allows successful reproduction even in new habitats with minimal inbreeding effects.

## Abstract

In general, when insects or other organisms invade a new habitat and establish a population, multiple individuals are required to arrive. When only a few individuals colonize a new area, they may fail to find mating partners, or inbreeding may produce genetically non-viable offspring. Monodontomerus obscurus is a parasitoid wasp that mainly attacks solitary bees. In Japan, this species was first recorded in 2000, and our field surveys showed that its parasitism rate has increased in recent years. By developing genetic markers and examining population structure and genetic variation, we found evidence that a population was founded by very few females, most likely a single female. Gregarious parasitoid wasps in the superfamily Chalcidoidea, including the present species, commonly mate with close relatives soon after emergence. This mating system may allow them to reproduce without searching for mates even in a new habitat, and to be less affected by the negative effects of inbreeding even after population establishment by a single individual.

How many founders are required for insects and other organisms to establish new populations is a fundamental question in invasion biology. We investigated the population establishment process of a parasitoid wasp, Monodontomerus sp., which was first recorded in Japan in 2000. Field surveys conducted in this study showed that the parasitism rate has been increasing in recent years. Morphological and molecular analyses suggested that the parasitoid species is M. obscurus, or a closely related lineage derived from it, which newly invaded Japan. To examine genetic variation during the early stage of invasion, we developed microsatellite DNA markers and conducted population genetic analyses. The results revealed extremely low genetic diversity: most loci were monomorphic, polymorphism was restricted to loci with long repeat motifs, and the allele frequencies of these loci were dominated by single alleles. A minimum spanning network based on microsatellite genotypes exhibited a star-like pattern. These results based on genome-wide microsatellite data indicate that the present population was founded by very few individuals, most likely a single female or an effectively single genetic lineage, and novel genotypes arose through post-invasion mutations. Our study provides rare empirical evidence for single-female founding under natural conditions, and highlights how species-specific life-history and genetic systems can enable successful invasion despite extreme bottlenecks.