Induction of Gametogenesis in the Cnidarian Endosymbiosis Model Aiptasia sp

Abstract

Endosymbiosis is widespread among cnidarians and is of high ecological relevance. The tropical sea anemone Aiptasia sp. is a laboratory model system for endosymbiosis between reef-building corals and photosynthetic dinoflagellate algae of the genus Symbiodinium. Here we identify the key environmental cues to induce reproducible spawning in Aiptasia under controlled laboratory conditions. We find that simulating a lunar cycle with blue-wavelength light is necessary to promote abundant gamete production and synchronous release in well-fed animals. Sexual reproduction rates are genetically determined and differ among clonal lines under similar conditions. We also find the inverse difference in rates of asexual reproduction. This study provides the requisite basis for further development of the Aiptasia model system, allowing analysis of basic cellular and molecular mechanisms in the laboratory as well as investigations of broad questions of ecological and evolutionary relevance.

Figure 1. Symbiosis throughout the cnidarians and the anthozoan life cycle.

(a) Phylogenetic tree of the major metazoan clades, with phyla shown in bold. Cnidarians represent a sister group to the bilaterians and are at the base of metazoan evolution. Symbiosis with dinoflagellates and green algae occur in species throughout the classes Medusozoa and Anthozoa, the latter of which contains Aiptasia sp. Illustrations were drawn by Stephanie Guse and are used with permission. (b) Overview of Aiptasia life cycle showing dual reproductive modes. *metamorphosis and settlement in the laboratory have not yet been reported, and as such remains an active experimental area.

Figure 2. Characterization of Aiptasia clonal lines CC7 (male), F003 (female), and H2 (female).

(a) Clonal lines appear morphologically similar, with minor variation in coloration and body size. (b) Maximum likelihood tree of ~2 kb concatenated SCAR genotyping markers showing the relationship of the three clonal lines to twelve field-sampled Aiptasia sp. individuals grouped into two genetic networks. Clonal lines are indicated by name; field-sampled individuals are named by their sampling location, with the second animal from the same location designed by’. Shown also are the endogenous Symbiodinium spp. (clades A-C and strain names) hosted by Aiptasia in the given genetic networks and in the three laboratory clonal lines; F003 symbiont characterization from this study.

Figure 3

Induction of spawning in Aiptasia. (a) Schematic of initial spawning induction conditions during two consecutive 29-day artificial lunar cycles. Temperature and moon cue indicated; color of the circle refers to color of the simulated moon cue (see text). (b,c) Spawning synchronicity and efficiency in [F003 x CC7] couples (b) and [H2 x CC7] couples (c) under the induction conditions in (a).(d) Schematics of various spawning induction conditions A-D for a direct comparison of spawning cues in [F003xCC7] couples. (e) Number of spawning events by five couples in each of the four induction conditions in (d) over one 29-day artificial lunar cycle. Error bars are standard deviations. (f) Quantification of total larvae per condition produced during the larvae spawning events shown in (e).

Figure 4. Line-specific differences in reproduction between Aiptasia female clonal lines.

(a) Quantification of larvae and egg production by [F003 x CC7] and [H2 x CC7] couples in two cycles of spawning induction in condition A from Fig. 3d. E denotes occurrence of eggs; numbers indicate approximate quantification of larvae. (b) Cross-sections of longitudinally dissected anemones from the three clonal lines shows many bulbous gonads within mesentery tissues. (c) Weekly quantification of asexual reproduction via pedal laceration in the three clonal lines over five weeks. n = five individuals per clonal line; error bars are standard deviations.