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.