A striking abundance of cryptic and pseudocryptic species was highlighted in marine invertebrates by a number of studies, the majority of which focused however on cryptic diversity as such, without inferring on evolutionary drivers involved in organism diversification. The widespread polychaete Syllis gracilis Grube, 1840 was identified as a possible species complex by MALTAGLIATI et al. (2000), who identified Mediterranean marine and brackish populations as putative cryptic species on the basis of allozyme data. More recently, sequence data allowed to consider the allegedly cosmopolitan S. gracilis as a paraphyletic species complex, including at least three distinct Mediterranean lineages. To understand the role of biogeographical and ecological factors in the differentiation of the complex, populations of S. gracilis were sampled from eleven Mediterranean localities, and a combined morphological and molecular characterisation was carried out. Multivariate and univariate analyses on the morphological dataset highlighted that the majority of traits are fixed at population level, but it is possible to distinguish two morphotypes, one with short cirri, associated with intertidal coralline algae, and one with long cirri occurring on Sabellaria reefs and in brackish environments. On the other hand, 16S rDNA sequences highlighted the occurrence of three separated lineages, of which only two correspond to those identified by Álvarez-Campos et al. (2017). Therefore, S. gracilis includes at least four cryptic lineages. Molecular lineages did not show any correspondence with morphological diversity, and natural populations of S. gracilis usually included at least two cryptic lineages, without a clear distinction between marine and brackish-water populations. This outcome suggests that the relationship between the molecular pattern detected and ecological and biogeographical features is not straightforward. The observed morphological pattern is most likely due to local adaptations and phenotypic plasticity, while the separation between molecular lineages is possibly due to ancient geological events. The diversity and evolution of marine invertebrates is shaped by a number of different processes working and interacting at different spatial and temporal scales. The reconstruction of evolutionary processes should therefore take into consideration as many different evidence lines as possible. Although the inconsistent pattern detected make the integration of morphological and molecular patterns difficult, these patterns are the consequence of unpredictable interactions between different evolutionary drivers, thus representing an interesting case of irreducible evolutionary complexity in marine systems.
Evolutionary complexity in marine invertebrates: the case of the Syllis gracilis (Annelida: Syllidae) species complex
Mikac B.;
2019
Abstract
A striking abundance of cryptic and pseudocryptic species was highlighted in marine invertebrates by a number of studies, the majority of which focused however on cryptic diversity as such, without inferring on evolutionary drivers involved in organism diversification. The widespread polychaete Syllis gracilis Grube, 1840 was identified as a possible species complex by MALTAGLIATI et al. (2000), who identified Mediterranean marine and brackish populations as putative cryptic species on the basis of allozyme data. More recently, sequence data allowed to consider the allegedly cosmopolitan S. gracilis as a paraphyletic species complex, including at least three distinct Mediterranean lineages. To understand the role of biogeographical and ecological factors in the differentiation of the complex, populations of S. gracilis were sampled from eleven Mediterranean localities, and a combined morphological and molecular characterisation was carried out. Multivariate and univariate analyses on the morphological dataset highlighted that the majority of traits are fixed at population level, but it is possible to distinguish two morphotypes, one with short cirri, associated with intertidal coralline algae, and one with long cirri occurring on Sabellaria reefs and in brackish environments. On the other hand, 16S rDNA sequences highlighted the occurrence of three separated lineages, of which only two correspond to those identified by Álvarez-Campos et al. (2017). Therefore, S. gracilis includes at least four cryptic lineages. Molecular lineages did not show any correspondence with morphological diversity, and natural populations of S. gracilis usually included at least two cryptic lineages, without a clear distinction between marine and brackish-water populations. This outcome suggests that the relationship between the molecular pattern detected and ecological and biogeographical features is not straightforward. The observed morphological pattern is most likely due to local adaptations and phenotypic plasticity, while the separation between molecular lineages is possibly due to ancient geological events. The diversity and evolution of marine invertebrates is shaped by a number of different processes working and interacting at different spatial and temporal scales. The reconstruction of evolutionary processes should therefore take into consideration as many different evidence lines as possible. Although the inconsistent pattern detected make the integration of morphological and molecular patterns difficult, these patterns are the consequence of unpredictable interactions between different evolutionary drivers, thus representing an interesting case of irreducible evolutionary complexity in marine systems.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
