Abstract
Revealing the mechanisms of life cycle changes is critical for understanding the processes driving hydrozoan evolution. Our analysis of mitochondrial (COI, 16S) and nuclear (ITS1 and ITS2) gene fragments resulted in the discovery of unique polymorphism in the life cycle of Sarsia lovenii from the White Sea. This polymorphic species exhibits two types of gonophores: hydroids produce both free-swimming medusae and attached medusoids (phenotypic polymorphism). Our phylogenetic analysis revealed the intrinsic genetic structure of S. lovenii (genetic polymorphism). Two haplogroups inhabiting the White Sea differ in their reproductive modes. Haplogroup 1 produces attached medusoids, and haplogroup 2 produces free-swimming medusae. Our experiments indicated the possibility of free interbreeding between haplogroups that likely is a rare event in the sea. We propose that inter-haplogroup crossing of S. lovenii in the White Sea may be limited by discordance in periods of spawning or by spatial differences in habitat of spawning specimens. Our finding can be interpreted as a case of nascent speciation that illustrates the patterns of repeated medusa loss in hydrozoan evolution. Life cycle traits of S. lovenii may be useful for elucidating the molecular mechanisms of medusa reduction in hydrozoans.
Highlights
Revealing the mechanisms of life cycle changes is critical for understanding the processes driving hydrozoan evolution
Since the medusa stage has not yet been described for S. lovenii, we provide a detailed description of this stage for the first time using the description of S. tubulosa medusae by P
As a result of molecular analysis (Supplementary Information Table S1) and long-term sporadic field observations of medusae and hydroids, we propose a scheme of the S. lovenii life cycle in the White Sea (Fig. 6)
Summary
Revealing the mechanisms of life cycle changes is critical for understanding the processes driving hydrozoan evolution. Louis Agassiz (1849) designated similar northwest Atlantic hydrozoans as Sarsia mirabilis and supposed the plasticity of its life cycle[24] He indicated that Coryne (Sarsia) mirabilis hydroids produce medusa in the early spring, but they produce attached medusoids at the end of the budding season in late spring[25]. He suggested the same life cycle for European hydrozoans and considered S. ramosa (=S. lovenii) to likely be only the phase assumed by S. sarsii (=S. tubulosa) towards the end of the breeding season[26,27]. Our main goal was to validate the species identity of attached medusoids and free-swimming medusae of Sarsia in the White Sea and to verify a hypothesis about S. lovenii life cycle polymorphism
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