Invasive Daphnia pulex/pulicaria in NZ
In this study, we identified two independent invasions of Daphnia pulex/pulicaria on the South and North Island of NZ. Phylogenetic data from mitochondrial genomes revealed that both invasive Daphnia pulex/pulicaria originated from North America. We examinedDaphnia pulex/pulicaria populations from 13 lakes on the South Island, only to find an average of 24 synonymous substitutions (over a total of 7.05 million synonymous sites) among populations and all the populations share a single mitochondrial haplotype, suggesting that all South Island Daphnia pulex/pulicaria are decendants of a single clone. In addition, all of the South Island Daphnia pulex/pulicaria are homozygous at the Ldh locus and have >97% sites homozygous for D. pulicaria -specific nucleotides, indicating they are non-hybrid D. pulicaria . On the contrary, the North Island Daphnia pulex/pulicaria are heterozygous at the Ldh locus and have >96.6% sites heterozygous for D. pulicaria - and D. pulex -specific nucleotides, thereby suggesting a hybrid origin of the North Island population.
Both the South and North Island invasions involved obligately asexualDaphnia clones. The origin of asexuality for the North IslandDaphnia pulex/pulicaria is likely caused by hybridization betweenD. pulex and D. pulicria prior to the arrival of NZ (Innes and Hebert 1988), while that for the South Island D. pulicaria is still not clear. Obligately asexual D. pulex × pulicariahybrids are notoriously invasive, invading many African lakes within just a few decades (Mergeay et al. 2006), and invading Japan between 680 and 3400 years ago (So et al. 2015). In Africa, the invading D. pulex × pulicaria hybrids spread and replaced the native sexualD. pulex (Mergeay et al. 2006). This might be due to obligate asexuals avoiding the cost of producing males, although obligately asexual Daphnia are often capable of producing males (Lynch et al. 2008), and there are additional ecological matters to consider. For example, the subset of obligate asexuals that are successful are often generalists capable of outcompeting sexuals when environmental dispersal is limited (Baker 1965; Parker 1977; Lynch 1984).
Although the NZ lakes in our study have historically been inhabited by native NZ species of Daphnia (Burns et al. 2017), we could not find any native Daphnia species in our samples, which is likely due to them being completely displaced by just two invading clones from North America, neither of which shows significant evidence of adaptive divergence based on genome-wide sequence analyses. The invasive asexualDaphnia clones could be bearers of a serious pathogen for the native Daphnia , because complete displacement requires a fortuitous ecological advantage over presumably well-adapted, genetically variable, long-term residents of these lakes.