Alike but different: the evolution of the Tubifex tubifex species complex (Annelida, Clitellata) through polyploidization
- Equal contributors
1 Istituto Italiano di Tecnologia (IIT), via Morego 30, Genova 16163, Italy
2 CIBIO, Centro de Investigação em Biodiversidade e Recursos Genéticos, Campus Agrário de Vairão, R. Padre Armando Quintas, Vairão 4485-661, Portugal
3 Università degli Studi di Pavia, Dipartimento di Genetica e Microbiologia "A. Buzzati Traverso", Via Ferrata 1, 27100 Pavia, Italy
4 Universitá degli Studi di Milano, Dipartimento di Bioscienze, Via Celoria 26, 20133 Milano, Italy
BMC Evolutionary Biology 2014, 14:73 doi:10.1186/1471-2148-14-73Published: 2 April 2014
Tubifex tubifex is a widespread annelid characterized by considerable variability in its taxonomic characteristics and by a mixed reproductive strategy, with both parthenogenesis and biparental reproduction. In a molecular phylogenetic analysis, we detected substantial genetic variability among sympatric Tubifex spp. from the Lambro River (Milano, Italy), which we suggested comprise several cryptic species. To gain insights into the evolutionary events that generated this differentiation, we performed a cytogenetic analysis in parallel with a molecular assay. Approximately 80 cocoons of T. tubifex and T. blanchardi were collected and dissected. For each cocoon, we sequenced a fragment of the 16S rRNA from half of the sibling embryos and karyotyped the other half. To generate a robust phylogeny enabling the reconstruction of the evolutionary processes shaping the diversity of these sympatric lineages, we complemented our original 16S rRNA gene sequences with additional COI sequences.
The chromosome number distribution was consistent with the presence of at least six sympatric euploid chromosome complements (one diploid, one triploid, three tetraploids and one hexaploid), as confirmed by a FISH assay performed with an homologous 18S rDNA probe. All the worms with 2n = 50 chromosomes belonged to an already identified sibling species of T. tubifex, T. blanchardi. The six euploid sets were coherently arranged in the phylogeny, with each lineage grouping specimens with the same chromosome complement.
These results are compatible with the hypothesis that multiple polyploidization events, possibly enhanced by parthenogenesis, may have driven the evolution of the T. tubifex species complex.