Impairment of reproductive capabilities in three subsequent generations of asymmetric hybrids between Eisenia andrei and E. fetida from French, Hungarian and Polish laboratory colonies.

Barbara Plytycz,Agnieszka Podolak,Pawel Grzmil,Janusz Bigaj,Sebastian Hofman,Artur Osikowski,Aleksandra Rysiewska,Michael Schubert

doi:10.1371/journal.pone.0235789

Barbara Plytycz, Agnieszka Podolak + Show 6 more

Open Access

https://doi.org/10.1371/journal.pone.0235789

Copy DOI

Abstract

Eisenia andrei (Ea) and E. fetida (Ef) lumbricid earthworms are simultaneous hermaphrodites potentially capable of self-fertilization and hybridization. We have shown previously that reproductive isolation in these species is incomplete in Ea and Ef earthworms of French provenance, as viable offspring appeared in inter-specific pairs. Fertile asymmetric hybrids developed from Ea-derived ova fertilized by Ef-derived spermatozoa, as well as Ea or Ef specimens derived after self-fertilization (resulting from admixture of endogenously produced spermatozoa with sperm from a partner), but never Ef-hybrids from Ef-ova fertilized by Ea-spermatozoa. The latter appeared only in backcrosses of Ea-hybrids with the Ef. Here we show that these phenomena are not unique for French Ea/Ef earthworms, but are shared by earthworms from French, Hungarian, and Polish laboratory cultures. Semi-quantitative studies on fertility of Ea-derived hybrids revealed gradually decreasing numbers of offspring in three successive generations, more rapid in backcrosses with Ef than with Ea, and the absence of progeny in pairs of hybrids, despite the presence of cocoons in almost all pairs. Based on species specific mitochondrial and nuclear DNA sequences, we provide the first examples of two unique sterile hybrids with mitonuclear mismatch and potential mitonuclear incompatibility among offspring of one of the hybrid+Ef pairs. Earthworms from the investigated populations did not reproduce when kept from hatching in isolation or with representatives of Dendrobaena veneta but started reproducing upon recognition of a related partner, such as Ea, Ef or their hybrids. The existence of Ea or Ef specimens among offspring of hybrid+Ea/Ef pairs might be explained either by partner-induced self-fertilization of Ea/Ef or hybrid-derived ova, or by cross-fertilization of Ea/Ef /hybrid ova by partner-derived spermatozoa; the latter might contribute to interspecific gene introgression.

Highlights

The eco-physiologically similar hermaphroditic earthworms, ‘red worms’ Eisenia andrei (Ea) and ‘tiger worms’ Eisenia fetida (Ef), were originally considered as pigmentation morphs of E. fetida, and later as subspecies
The French Ea and Ef specimens do not reproduce when kept in isolation from a hatchling stage [11], whereas virgin Spanish earthworms of these species are capable of uniparental reproduction [12]
The aim of the present laboratory investigation was to test whether asymmetrical hybridization between Ea and Ef earthworms, partner-induced self-fertilization, and lack of reproduction in virgin isolated earthworms are unique features for earthworms of French provenance, or if they are more general phenomena. To answer these questions we investigated earthworms from French, Hungarian, and Polish laboratory stocks and 1) looked for signs of reproduction of virgin specimens kept in isolation; 2) joined adult virgin earthworms for 3–4 months in interspecific Ea+Ef pairs and genotyped their offspring; 3) repeated the same procedures with the hybrids (H) of the first, second and third generations either backcrossed with Ea or Ef or joined with other hybrids

Summary

Introduction

The eco-physiologically similar hermaphroditic earthworms, ‘red worms’ Eisenia andrei (Ea) and ‘tiger worms’ (or ‘brandlings’) Eisenia fetida (Ef), were originally considered as pigmentation morphs of E. fetida, and later as subspecies. They are treated as two distinct species, Ea and Ef, with species-specific mitochondrial and nuclear DNA sequences [1,2,3], the latter with Ef1 and Ef2 mitochondrial lineages [4]. The French Ea and Ef specimens do not reproduce when kept in isolation from a hatchling stage [11], whereas virgin Spanish earthworms of these species are capable of uniparental reproduction [12] These apparently contradictory observations suggest the existence of various modes of reproductive isolation operating in geographically distant earthworms within Eisenia sp. Further studies on the breeding biology of these hermaphroditic species capable of self-fertilization are pertinent

Methods

Results

Conclusion