The existence of fertile hybrids of closely related model earthworm species, Eisenia andrei and E. fetida

Abstract

Lumbricid earthworms Eisenia andrei (Ea) and E. fetida (Ef) are simultaneous hermaphrodites with reciprocal insemination capable of self-fertilization while the existence of hybridization of these two species was still debatable. During the present investigation fertile hybrids of Ea and Ef were detected. Virgin specimens of Ea and Ef were laboratory crossed (Ea+Ef) and their progeny was doubly identified. 1 –identified by species-specific maternally derived haploid mitochondrial DNA sequences of the COI gene being either ‘a’ for worms hatched from Ea ova or ‘f’ for worms hatched from Ef ova. 2 –identified by the diploid maternal/paternal nuclear DNA sequences of 28s rRNA gene being either ‘AA’ for Ea, ‘FF’ for Ef, or AF/FA for their hybrids derived either from the ‘aA’ or ‘fF’ ova, respectively. Among offspring of Ea+Ef pairs in F1 generation there were mainly aAA and fFF earthworms resulted from the facilitated self-fertilization and some aAF hybrids from aA ova but none fFA hybrids from fF ova. In F2 generation resulting from aAF hybrids mated with aAA a new generations of aAA and aAF hybrids were noticed, while aAF hybrids mated with fFF gave fFF and both aAF and fFA hybrids. Hybrids intercrossed together produced plenty of cocoons but no hatchlings independently whether aAF+aAF or aAF+fFA were mated. These results indicated that Ea and Ef species, easy to maintain in laboratory and commonly used as convenient models in biomedicine and ecotoxicology, may also serve in studies on molecular basis of interspecific barriers and mechanisms of introgression and speciation. Hypothetically, their asymmetrical hybridization can be modified by some external factors.

Asymmetrical hybridization and gene flow between Eisenia andrei and E. fetida lumbricid earthworms

“…&: atypical Mug-negative hybrid fFA 149/194 Mn; #: atypical MUG-positive specimen fFF 158/190 . Genebank accession numbers are given in [ 18 ]. Inset: Examples of fluorescence spectra of excitation (left) and emission (right) in coelomic fluid of Mp (orange solid lines) and Mn (blue dotted lines) specimens.…”

“…The main analyses of coelomic fluid were performed on 46 out of 158 descendants of laboratory-paired M-positive Ea and M-negative Ef specimens genotyped previously [ 18 ]. In short, during previous investigations the pairs of freshly hatched earthworms were cultured until cocoon production/reproduction.…”

“…The aAF hybrids mated with Ea gave a new generation of Ea and aAF hybrids, while mated with fFF gave fFF, aAF, and sterile fFA hybrids. Pairs of hybrids, both aAF and fFA, produced plenty cocoons but no hatchlings [ 18 ].…”

“…For main experiments, 46 genetically identified aAA, fFF, aAF, or fFA specimens from previous study [ 18 ] of similar body weights (X = 0.77+0.18 g), were used for analyses of coelomic fluid.…”

“…Among F1offspring there were self-fertilized Ea (aAA), Ef (fFF), and cross-fertilized fertile hybrids (aAF) derived from Ea ova; the aAF hybrids mated with Ea gave new generation of Ea and hybrids, and while mating with Ef gave Ea, Ef, aAF and sterile fFA hybrids derived from Ef ova. Using the methods of the combined mitochondrial and nuclear markers we detected on the Ea branch of the COI-based phylogram both the ‘pure’ Ea specimens (aAA) and relatively common inter-specific hybrids (aAF), while on the Ef branch there were both ‘pure’ fFF specimens and a few sterile fFA hybrids [ 18 ].…”