Reproduction and the pheromonal regulation of sex type in fern gametophytes

Abstract

The fern life cycle includes a haploid gametophyte that is independent of the sporophyte and functions to produce the gametes. In homosporous ferns, the sex of the gametophyte is not fixed but can vary depending on its social environment. In many species, the sexual phenotype of the gametophyte is determined by the pheromone antheridiogen. Antheridiogen induces male development and is secreted by hermaphrodites once they become insensitive to its male-inducing effect. Recent genetic and biochemical studies of the antheridiogen response and sex-determination pathway in ferns, which are highlighted here, reveal many similarities and interesting differences to GA signaling and biosynthetic pathways in angiosperms.

Keywords: GA biosynthesis; GA signaling; antheridiogen; ferns; sex determination.

FIGURE 1

The C. richardii life cycle. Typical of all homosporous ferns, the diploid sporophyte produces sporangia on the abaxial surface of the fronds. Each sporangium contains haploid spores that are released from the sporophyte and, in the case of C. richardii, can remain dormant but viable for more than 50 years. Each spore germinates and develops as a male or hermaphroditic gametophyte depending on the presence or absence of antheridiogen. When mature, sperm are released and swim to the egg. The young sporophyte remains dependent on the gametophyte for a short period of time.

FIGURE 2

The antheridiogen response in C. richardii. A single spore always develops as a hermaphrodite when grown in the absence of A_CE. The hermaphrodite consists of a single sheet of cells with a distinct multicellular meristem that forms a meristem notch and multiple archegonia that develop adjacent to the meristem notch, which are highlighted in the SEM (boxed area of the hermaphrodite). Hermaphrodites secrete A_CE; in the presence of A_CE, spores develop as males. The male lacks a meristem and almost all cells differentiate as antheridia. The SEM shows six antheridia, each having a ring cell and a cap cell that pops open to release sperm. When a male gametophyte is transferred to media lacking A_CE, some cells divide and begin to form a hermaphroditic prothallus. The “switched” male shown is forming three such prothalli. mn: meristem notch; ar: archegonia; cc: cap cell; rc: ring cell.

FIGURE 3

A comparison of the GA signaling pathway in angiosperms and the sex-determining (SD) pathway in C. richardii. The SD pathway in C. richardii is based solely on the epistatic interactions among sex-determining mutants but it is consistent with recent molecular and biochemical studies in the fern L. japonicum. T bars represent repressive events whereas arrows indicate activating events.