The Affect of Ultra Violet Radiation on Sex Determination in Ceratopteris Richardii: a Comparison Between Wild-Type and Her1

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Abstract:
Ultraviolet (UV) radiation and its detrimental effects on plant life have been widely researched. This research studies the effects of UV radiation on sex-determination in Ceratopteris richardii. Sex determination in Ceratopteris richardii is determined by antheridiogen, a pheromone that promotes maleness. In this experiment, UV radiation was used on the wild type and mutants Her1, which are insensitive to the male-inducing pheromone (does not form male gametophytes). Ceratopteris richardii spores were exposed to UV radiation at different stages during their life cycle. Wild type gametophytes treated with UV radiation until germination and sexual maturity showed an increase of hermaphrodites as compared to wild type gametophytes only exposed to white light. Her1 mutants treated with UV radiation until germination showed no change in expected sex ratios. However, her1 mutants exposed to UV radiation until sexual maturity showed a large discrepancy in normal sex ratios and produced a huge increase of males. These results show that UV radiation does have an effect on sex determination in Ceratopteris richardii, and perhaps other plants as well.

Introduction

Ceratopteris richardii, also known as C-Fern, start out as haploid spores. After four to six days, germination takes place and rhizoids, a root-like filament which absorbs nutrition, appear. It begins to develop into a sexually mature gametophyte, an ameristic male or hermaphrodite with a well-defined meristic region. Figure 1: C-Fern Life Cycle

Source: D. Maxwell. “C-Fern Genetics”
The sexual fate of the gametophyte depends if the pheromone antheridiogen (ACE) is present in the early stages of development, approximately three to four days after spore inoculation. ACE, which is thought to be a form of gibberellin, promotes maleness in Ceratopteris richardii gametophytes and must be present during the entire duration of cell division to allow the production of antheridia. ACE is responsible for both the initiation and maintenance of the male program of expression in Ceratopteris richardii and therefore acts as a repressor for meristem and archaegonia formation. This mechanism of sexual determination allows for the ratio of males and hermaphrodites to be dependent upon the amount of ACE within the culture. The HER (hermaphroditic), FEM1 (feminization) and TRA1 (transformer) genes are directly affected by ACE. HER genes are necessary for ameristic male development and are likely to encode factors that make up the ACE receptor and/or signal transduction pathway [Eberle and Banks, 1996]. The expression of TRA1 allows meristem and archegonia development in Ceratopteris richardii while FEM1 allows for antheridia development. ACE signals are commonly blocked by the hormone, abscisic acid (ABA). ABA is known to block antheridiogen response in undifferentiated cells of the gametophyte, indicating that ABA may repress antheridia development [Banks, Hickok, and Webb, 1993]. Figure 2: ACE Pathway

Source: Eberle and Banks, 1996.

If ACE is absent during the stage of cell division, the gametophyte will develop as a hermaphrodite, an action that is irreversible even if exposed to ACE in later stages. The HER gene is not active while TRA and FEM are expressed and the gametophyte develops an archegonia, antheridia and a meristem. In this case, TRA promotes meristem and archegonia development and FEM promotes antheridia development (Figure 2).

However, if ACE is present during the beginning parts of the stage but is withdrawn later, the undifferentiated cells of the male begin to divide and form a hermaphrodite. The HER and TRA1 genes would remain unexpressed while FEM1 will be expressed (Figure 2). In the continuous presence of ACE, ameristic males containing antheridia, male sexual organs each containing 16 sperm [Warne and Hickok, 2008], are formed. In this case, HER and FEM1 genes are expressed and repress TRA1 (Figure 2). After sexual maturity occurs,...
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