Ecological stoichiometry of environmental sex determination in the fern Ceratopteris richardii
In the homosporous fern Ceratopteris richardii, sex is not genetically fixed but instead environmentally determined during development. As in many ferns, environmental sex determination (ESD) is initially mediated by the gibberellin-like substance antheridiogen (Ace), which promotes the production of ameristic males (lacking a lateral meristem). Meristic gametophytes continue growth and can potentially remain asexual or become male, female or hermaphroditic. Nutrient poor environments are theorized to limit growth below a threshold size or cause physiological stress favoring sexual differentiation as males, and reduce average female size.
To test for these effects, C. richardii gametophytes were grown in two nutrient arrays encompassing ecologically relevant nutrient levels. One array had four levels each of nitrogen (N) and phosphorous (P) resulting in 16 molarities with 13 unique N:P stoichiometries. A second array had 6μM glucose (C) added to the same 16 N:P array. The atomic ratios of the least and most concentrated media were N:P = 1 and C:N:P = 5:48:4, respectively. Nutrient effects on Ace-based and meristic plant ESD were evaluated.
We found no significant main effect of molarity, N:P stoichiometry, or their interaction on the percentage of ameristic males formed in the absence of glucose during in the Ace-based stage of gametophyte development. In the presence of glucose, the interaction between molarity and stoichiometry significantly influenced percent ameristic males. Consequently, the antheridiogen response in C. richardii was not nutrient-dependent unless ecologically relevant amounts of glucose were present.
Nutrient effects on meristic plant phenotypes were examined in a subset of the treatments (four levels of N at P = 1, +/- C). Gametophytes at the same stage of archegonial development were not significantly different in area, regardless of nitrogen molarity. Additionally, glucose influenced the size threshold for archegonial development, causing female plants to reach the same stage of sexual maturity at either a smaller or larger size than their non-glucose counterparts. In some cases, glucose may be acting as a signal for the onset of sexual development, prompting sexual maturity in females at a smaller size threshold. The results suggests that carbon availability could be an important environmental variable for determining how gametophytes with ESD respond to variation in the molarity and stoichiometry of nitrogen and phosphorous.