Sexual Plasticity and Self-Fertilization in the Sea Anemone Aiptasia diaphana

Loya Y (2010) Sexual Plasticity and Self-Fertilization in the Sea Anemone Aiptasia diaphana. PLoS ONE 5(7): e11874. doi:10.1371/journal.pone.0011874 Sexual Plasticity and Self-Fertilization in the Sea Anemone Aiptasia diaphana Ami Schlesinger 0 Esti Kramarsky-Winter 0 Hanna Rosenfeld 0 Rachel Armoza-Zvoloni 0 Yossi Loya 0 Ryan L. Earley, University of Alabama, United States of America 0 1 Department of Zoology, George S. Wise Faculty of Life Sciences, Tel Aviv University , Tel Aviv , Israel , 2 Israel Oceanographic and Limnological Research, National Center for Mariculture , Eilat , Israel Traits that influence reproductive success and contribute to reproductive isolation in animal and plant populations are a central focus of evolutionary biology. In the present study we used an experimental approach to demonstrate the occurrence of environmental effects on sexual and asexual reproduction, and provide evidence for sexual plasticity and inter-clonal fertilization in laboratory-cultured lines of the sea anemone Aiptasia diaphana. We showed that in A. diaphana, both asexual reproduction by pedal laceration, and sexual reproduction have seasonal components. The rate of pedal laceration was ten-fold higher under summer photoperiod and water temperature conditions than under winter conditions. The onset of gametogenesis coincided with the rising water temperatures occurring in spring, and spawning occurred under parameters that emulated summer photoperiod and temperature conditions. In addition, we showed that under laboratory conditions, asexually produced clones derived from a single founder individual exhibit sexual plasticity, resulting in the development of both male and female individuals. Moreover, a single female founder produced not only males and females but also hermaphrodite individuals. We further demonstrated that A. diaphana can fertilize within and between clone lines, producing swimming planula larvae. These diverse reproductive strategies may explain the species success as invader of artificial marine substrates. We suggest that these diverse reproductive strategies, together with their unique evolutionary position, make Aiptasia diaphana an excellent model for studying the evolution of sex. - Funding: This work was funded by RAMOT (Horowitz Foundation) Tel Aviv University. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. Traits that influence reproduction in animal and plant species have been a central focus of evolutionary biology since Darwin. Therefore, considering their evolutionarily basal position, the Anthozoa (Cnidaria) provide important models for further understanding processes affecting reproductive strategies in the eumetazoan (i.e., cnidarian-bilaterian) ancestor and in modern vertebrates [1]. Many anthozoans reproduce through both asexual and sexual means [2]. In general, populations of organisms that rely on asexual reproduction are ultimately characterized by the number of genetic individuals (genets) being lower than the number of actual individuals (ramets) in an area [3]. Indeed at times such local populations may be asexual products of single clones. If this is the case, that the possibility of fertilization occurring within a genet or clone (defined here as self-fertilizing or selfing) may be an important reproductive option for such organisms. The evolution of these species may be enhanced by the meiosis and recombination that occur during sexual reproduction [46] while asexual reproduction may enhance their successful occupation of new spaces [7]. In most anthozoans, successful sexual reproduction occurs when either both eggs and sperm are shed into the water column, where fertilization and development occur, or when only sperm are released and fertilization is internal [2]. Spawning events may occur as single yearly events or repeatedly throughout the year; as single species or mass multispecies events [89]. Thus, genetic variability may be attained via either reproductive isolation or possible hybridizations [10]. Sea anemones (Cnidaria: Anthozoa) exhibit a variety of patterns of sexual reproduction and breeding patterns, even among species of the same genus [11]. One such anemone is the sea anemone Aiptasia diaphana from the eastern Mediterranean Sea. This species is found primarily in isolated fouling communities, making it an excellent model for studying how reproductive strategies may be instrumental in establishing and maintaining new populations. We therefore, undertook to evaluate its reproductive modes in order to understand how they may reflect its dispersal strategy. Natural gametogenic period and sexual character In field populations of Aiptasia diaphana (Figure 1) gametogenesis occurred between April and August with a peak in the percent reproductive individuals (see Figure S1), as apparent from the presence of gametes in the histological sections analyzed (Figure 2). During the gametogenic season randomly sampled specimens were either male (n = 14) or female (n = 11), or did not possess gametes (n = 55). No hermaphrodites were found in samples of naturally occurring A. diaphana. Asexual reproduction rate In laboratory culture, the rate of asexual reproduction was affected by controlled seasonal variations. Under JuneAugust (summer) temperaturephotoperiod conditions genets propagated ca. one order of magnitude more ramets (time = 77 d, n = 255627) than under December March (winter) conditions (t = 78 d, n = 23612). The rate of asexual reproduction differed significantly between the summer and winter treatments (2 way ANOVA and Tukeys HSD test, df = 1 Fseason 83.61; p,0.05). Accordingly, each founder genet that produced 255 ramets (individuals resulting from asexual reproduction) during summer and in turn produces at least 20 additional ramets during winter can, theoretically, produce over 5,000 ramets (20winterX 255summer) over a one-year period. Sexual character of controlled laboratory-reared genets Microscopic analysis of ramets (n = 304) sampled from six different laboratory-reared genet lines (G1 G6), each founded by one individual of a known sex, revealed that the male phenotype was preserved throughout the experiment in one genet (G5) (Table 1, Figure 2). The other five genets gave rise to both female and male phenotypes, with a skewed female/male ratio in favor of the founder sex (Table 1). In addition, ramets derived from one female genet (G2), included not only males and females but also seven hermaphroditic individuals (Table 1). Fertilization Gametes spawned by G2 (n = 3 spawn dates and G3 (n = 2 spawn dates) genet lines self-fertilized. Similarly, gametes spawned by G1cross-fertilized with gametes of G5 (n = 4 spawn dates). Zygotes derived from selfing and out-crossing developed into swimming planula larvae. The embryos underwent cha (...truncated)