Ontogeny reversal and phylogenetic analysis of Turritopsis sp.5 (Cnidaria, Hydrozoa, Oceaniidae), a possible new species endemic to Xiamen, China

Abstract

Ontogeny reversal, as seen in some cnidarians, is an unprecedented phenomenon in the animal kingdom involving reversal of the ordinary life cycle. Three species of Turritopsis have been shown to be capable of inverted metamorphosis, a process in which the pelagic medusa transforms back into a juvenile benthic polyp stage when faced with adverse conditions. Turritopsis sp.5 is a species of Turritopsis collected from Xiamen, China which presents a similar ability, being able to reverse its life cycle if injured by mechanical stress. Phylogenetic analysis based on both 16S rDNA and cytochrome c oxidase subunit I (COI) genetic barcodes shows that Turritopsis sp.5 is phylogenetically clustered in a clade separate from other species of Turritopsis. The genetic distance between T. sp.5 and the Japanese species T. sp.2 is the shortest, when measured by the Kimura 2-Parameter metric, and the distance to the New Zealand species T. rubra is the largest. An experimental assay on the induction of reverse development in this species was initiated by cutting medusae into upper and lower parts. We show, for the first time, that the two dissected parts have significantly different potentials to transform into polyps. Also, a series of morphological changes of the reversed life cycle can be recognised, including medusa stage, contraction stage I, contraction stage II, cyst, cyst with stolons, and polyp. The discovery of species capable of reverse ontogeny caused by unfavorable conditions adds to the available systems with which to study the cell types that contribute to the developmental reversal and the molecular mechanisms of the directional determination of ontogeny.

Keywords: China; Hydroid; Medusa; Reverse development; Turritopsis.

Figure 1. Maps of sampling site.

(A) A panorama of Fujian and Taiwan beside both sides of the Taiwan Strait in southeastern China. The sample collection site is represented in the dotted rectangle. (B) The exact sample collection site at the edge of Xiamen Island is marked with a pink dot.

Figure 2. Phylogenetic analysis based on 16S rDNA partial sequences.

Maximum Likelihood tree of the genus Turritopsis was based on mitochondrial 16S rDNA genes. A total of 46 16S rDNA sequences were analyzed. The Turritopsis rubra represents nine sequences of T. rubra 16S rDNA genes in Genbank (Table S1). For each node, Maximum Likelihood (the first number), Neighbor Joining (the second number), and Maximum Parsimony (the third number) bootstrap supports are reported (1,000 replicates). Bootstrap values were only shown if they were greater than 65% on the ML tree. The resulting clades are shown close to the branches. The scale bar represents 2% sequence divergence. The red asterisks denote the new sequences produced in this study.

Figure 3. Phylogenetic analysis based on COI partial sequences.

Maximum Likelihood tree of the genus Turritopsis based on the COI mitochondrial gene. A total of 24 COI sequences were analyzed. For each node, Maximum Likelihood (the first number), Neighbor Joining (the second number), and Maximum Parsimony (the third number) bootstrap supports are reported (1,000 replicates). Bootstrap values were only shown if they were greater than 65% on the ML tree. The resulting clades were shown close to the branches. The scale bar represents 5% sequence divergence. The red asterisks denote the new sequences produced in this study.

Figure 4. Stages of reverse development in Turritopsis sp.5 from Xiamen, China.

(A) A mature medusa. Scale bar: 2 mm. (ep, epidermis; ma, manubrium; rac, radial canal; ric, ring canal; tb, tentacle bulb; te, tentacle) (B) Upper part of medusa after being cut with inverted bell. Scale bar: 500 µm. (C) Four-lobe clover shaped upper part cut from the medusa. Scale bar: 1 mm. (D) Degeneration of the upper part of the medusa after cut. Scale bar: 500 µm. (E) Contraction stage I: tentacles co ntracted after the medusa was cut. Scale bar: 1 mm. (te, tentacle) (F) Contraction stage II: lower part of medusa after cut began to fuse. Scale bar: 500 µm. (G) A ball-like cyst formed. Scale bar: 500 µm. (H) Two outgrowths of stolons. Scale bar: 500 µm. (st, stolon) (I) A rejuvenated polyp. Scale bar: 200 µm (st, stolon).