Environmental Control of Sex Determination
Chih Hao Huang
Introduction
In many organisms, sex are determined by their inheritance of sex chromosomes in developing embryo, such as human and grasshoppers (Hurney et al. 2016). But some other organisms are different, their development of sexual phenotype do not rely on chromosomes, but influence from environmental factor such as temperature or parasite invasion (Hurney et al. 2016).
One of the example is Ceratopteris richardii (C-Fern), which is a tropical homosporous fern, contains no sex chromosomes but can differentiate into hermaphrodite or male gametophyte (Hurney et al. 2016). The sorus on the leaf of C-Fern produce spores via meiosis (Hurney et al. 2016). During gametophyte stage, C-fern spores produce haploid gametes (sperm and/or egg) via mitosis; male gametophyte produce sperms and hermaphrodite gametophyte produce both sperms and egg (Hurney et al. 2016). Gametophytes germinate earlier are hermaphroditic, which are heart shaped with archegonia and antheridia; males are smaller and thumb shaped, surrounded by antheridia (Hurney et al. 2016). Sperm and egg will fertilize in to a diploid zygote, which is the beginning of sporophyte stage. Later the zygote will develop into an embryo by mitosis, and then the embryo will develop into a fern and the life cycle starts again (Hurney et al. 2016). By being a homosporous, C-Fern can self-fertilize with hermaphroditic gametophyte which promote survival; having males gametophyte produce sperms allow C-Fern to cross fertilize, which provide genetic variation and increased fitness (Hurney et al. 2016).
The goal of this experiment is to observe gametophyte population density influences sex determination in Ceratopteris richardii. In the experiment, specific quantity of C-Fern spores was mixed with distilled water in a test tube and dilute into five more tubes, lead to six tubes with different amount of C-Fern spores, which been spread evenly on 6 plates of agarose (Hurney et al. 2016). Plates were incubated for three weeks to allow spores to produce the gametophyte generation (Hurney et al. 2016). We predict that the percentage of male will increase as the population density increase or the percentage of male will increase as the population density increase until certain point, the percentage of male will not increase as the population density increase.
Results
The data are indicative of increase in percentage of male as a function of density until certain point (Figure 1). The percentage of male has a significant increase when the population density is below 20 (Figure 1). The value of R2 is 0.7412. There are three outliers (dish A, E and F) which are slightly off in gametophyte density that lower the R2-value from 0.9983 to 0.7412. The data we got from the experiment have the same trends as our second prediction.

Figure 1. The relationship between the population density of Ceratopteris richardii and the percentage of male gametophytes.
Discussion
Overall, the percentage of male gametophytes will increase as the population density increase, but further studies would be necessary to determine anything for certain. When the population density is low, hermaphroditic gametophyte were more favored, because hermaphroditic gametophyte provides higher survival; and as the population density was higher, male gametophyte were more favored, because cross fertilization can provide genetic variation, which leads to genetic diversity of offspring. Male gametophytes did not continue to increase indefinitely as overall density increased, if too much male gametophytes were produced, there will not be enough eggs to fertilize with sperms. For those outliers which are slightly off, they are relatively compact on one side of the dish, such as dish E. There are many hermaphroditic gametophyte on the top right part of the dish, which also most males were produced in those areas (Figure 2). The experiment shows that the population density (environmental condition) has an effect on the development of sexual phenotype in the gametophyte of C-Fern.

Figure 2. Dish E gametophyte distribution on plate.
Literature Cited
Hurney, C.A., A. Pesce, S.K. Babcock, and O. Hyman. 2016. Organisms, a laboratory manual for BIO 114, Fourteenth Edition. Hayden-McNeil Publishing, Plymouth, Michigan, USA.