Nuclear DNA recapitulates the cryptic mitochondrial lineages ofLumbricus rubellusand suggests the existence of cryptic species in an ecotoxological soil sentinel

Abstract

Mitochondrial DNA analysis has revealed two distinct phylogenetic lineages within the ecotoxological sentinel earthworm model Lumbricus rubellus Hoffmeister, 1843. The existence of these lineages could complicate ecotoxicological studies that use the species as a sentinel for soil contamination testing, as they may respond differently to contamination; however, as mitochondrial haplotypes are not always expected to segregate in the same way as chromosomal DNA in natural populations, we further investigated this issue by using nuclear DNA markers (microsatellites) to measure genetic diversity, differentiation, and gene flow in sympatric populations of the two L. rubellus lineages at two sites in South Wales. Our results show that sympatric populations of the two lineages are more genetically differentiated than geographically distant populations of the same lineage, and Bayesian clustering analysis revealed no evidence of gene flow between the lineages at either site. Additionally, DNA sequencing of these microsatellite loci uncovered substantial differentiation between lineages at homologous flanking regions. Overall our findings indicate a high degree of nuclear genetic differentiation between the two lineages of L. rubellus, implying reproductive isolation at the two study sites and therefore the potential existence of cryptic species. The existence of two cryptic taxa has major implications for the application of L. rubellus as an ecotoxicological sentinel. It may therefore be necessary to consider the lineages as separate taxa during future ecotoxicological studies.