The Rapid, Mass Invasion of New Zealand by North American Daphnia pulex/pulicaria

Non-indigenous Daphnia 'pulex' have been found in many lakes in New Zealand (NZ) in the past 20 years, suggesting a recent invasion. However, very little is known about the origin of invasive D. 'pulex', whether each lake is invaded by a single clone or multiple clones, whether the invasive clones are D. pulex or D. pulicaria, and whether they are obligately asexual clones or cyclical parthenogens. Furthermore, the source and time of arrival of the invasive genotype(s) are unclear. We address these questions by genomic sequencing Daphnia populations from 13 lakes in the South Island and one lake in the North Island, NZ. We found that all biallelic sites in these NZ populations have similar numbers of reads for the two parental alleles, suggesting that each NZ population is a single clone. Based on ~24,000 monomorphic lineage-specific markers, the invasive Daphnia in the South Island were found to be D. pulicaria, while those in the North Island are D. pulex/ pulicaria hybrids. Both the South and North Island Daphnia are phylogenetically clustered with North American D. pulicaria/pulex, thereby suggesting their North American origins. We found also that all South Island clones contain identical mitochondrial genomes, suggesting the origin and proliferation from a single founder clone, which we experimentally verified to be an obligate asexual. Estimates from molecular data imply a colonization time for the South Island clones of ~ 60 years ago, with a likely invasion route associated with the introduction of salmonids from North America.