Positive and negative frequency dependent parasitism in naturally co-occurring lineages of diploid sexual and polyploid asexual Lumbriculus variegatus

Polyploidisation is an important evolutionary force. It drives sympatric
speciation through reproductive isolation of different cytotypes, and
often leads to loss of sexual reproduction in polyploid lineages.
Polyploidisation and asexuality can change how other species engage in
ecological interactions with the polyploid lineage and may change
coevolutionary dynamics. Here, we quantified the phenotypic divergence in
the freshwater oligochaete worm Lumbriculus variegatus, the California
blackworm, among its co-occurring sexual diploid (Lineage II) and asexual
polyploid (Lineage I) lineages. We further investigated variation in
parasite communities and infection prevalence among sympatric and
allopatric diploid/polyploid populations. Ten out of 18 populations showed
co-existence of both lineages, with 7 populations harbouring only the
polyploid lineage. Both worm lineages hosted  endoparasitic
nematodes, an ectoparasitic rotifer, and one potentially symbiotic gut
ciliate. The parasite community similarity and overlapping size range of
diploid and polyploid worms points to the ecological similarity of the
worm lineages, despite the substantial ploidy and reproductive strategy
differentiation. Although parasite prevalence varied independently of worm
lineage, prevalence was associated with frequency of local cytotypes.
Specifically, the rotifer prevalence was highest on the rare local
cytotype, and nematode prevalence was highest on the common local
cytotype. These results suggest the presence of both positive and negative
frequency dependent parasitism, which may contribute to the co-existence
in the L. variegatus species complex.