Ceratopteris richardii, known as a C-fern has a lifecycle referred to as alteration of generations, which consist of neither haploid nor diploid dominant. C-ferns are homospours plants which are important in that they can produce hermaphrioditic gametophytes in order to be able to self fertilize. However, some of the homospourous C-ferns only produce male gametophytes .The life cycle of Ceratopteris richardii starts as a diploid sporophyte which then, by meiosis, produces haploid spores. These spores then undergo mitosis to produce a haploid gametophyte, which can be either hermaphrodite( producing eggs and sperm), or a male gametophyte (producing only sperm). Gametophytes then produce gametes by mitosis. The hermaphrodite gametophyte will produce both eggs and sperm,while, the male gametophyte will produce strictly sperm. The gametes from the hermaphrodite gametophyte fertilize one another producing a diploid zygote which grows by mitosis into a diploid sporophyte ready to start the cycle again (Lab Manual).
Chromosomal sex determination is determined by the inheritance of sex chromosomes, while, environmental sex determination is influenced by environmental factors such as temperature and parasite invasion(Lab Manual). Humans use the system of chromosomal sex determination by combining their X and Y chromosomes to produce a zygote determining the sexual phenotype in humans. Alligators use environmental sex determination when incubating their eggs.Temperature influences the sex of the offspring meaning; warmer incubating temperatures produce male development while cooler temperatures produce female development (Lab Manual).
Our hypothesis for the Ceratopteris richardii was that gametophyte population density influences sex determination. Our first prediction was that the percentage of male gametophytes would increase as the population density of the gametophytes increased. Our second prediction was that the percentage of male gametophytes would decrease as the population density of the gametophytes increased.
The graph of our results showed the percentage of male gametophytes increasing and then leveling off (Figure 1). This was our trend because most of the points followed this pattern, however; there was a potential outlier at point (68, 32.3). The distribution of gametophytes on the culture plates varied. Plates A and F had even distribution but were very crowed with gametophytes, while plates D and C were evenly distributed but had less gametophytes. Plate E had clumping of gametophytes in the top left corner and plate B had clumping the the bottom right corner.
Figure 1. The relationship between the population density and the percentage of male gametophytes.
The prediction that the percentage of male gametophytes would increase as the population density of the gametophytes increased, and the prediction that the percentage of male gametophytes would decrease as the population density of the gametophytes increased were not supported by our results. When the population density was low, the percentage of male gametophytes was relatively low because there would not be enough eggs to fertilize the sperm produced from both the hermaphrodite,and the sperm produced by the male gametophyte. When the population density was high, the percentage of male gametophytes was higher but still less than fifty percent because of the same reasons.The data from our results did support our hypothesis that the gametophyte population influences sex determination in the C-fern because, the higher the population density, the more level the percentage of male gametophytes became.
The reason for this leveling out at the top of the graph was due to; if the percentage of male gametophytes kept increasing linearly, then there would be too many male gametophytes and not enough eggs being produced from the hermaphrodite gametophytes to be fertilized causing an unbalanced population. The outlier pertained to this because the population density was so high but the percentage of male gametophytes was relatively close to the other points supporting that male gametophyte production does not increase as population density increases. Even thought the population density was so high, the percentage of male gametophytes was relatively the same as the other points because the hermaphrodite gametophytes were trying to increase in number so that they could produce enough eggs to be able to self fertilize and reproduce with the male gametophytes, causing a leveling off in the graph.
C-Ferns might have evolved this system of sex determination for many reasons. Being a hermaphrodite allows them to self fertilize, so by having this ability,C-ferns can produce eggs when there is sperm, so they always have the ability to reproduce. Unlike hermaphrodites, having separate sexes does not always guarantee you a parter so there is not guarantee in reproduction. Cross fertilization and self fertilization are important when discussing variation. Cross fertilization allows for different genetic material to be combined causing variation.
Variation allows natural selection to act on and therefore, plants that are better adapted to the environment survive. Self Fertilization allows plants that have the adapted survival gene to self fertilize, producing less variation but more of sustainable plant. This relates to the offspring produced by hermaphrodites because they mostly self fertilize so they are producing more of themselves but less variation. The offspring produced by separate sexes produces more variation but stands less of a chance against natural selection because the hermaphrodites are reproducing well adapted offspring while the separate sexes are producing a new combination of genetics.
A Labratory Manual for BIO 114. 2011. Environmental Control of Sex Determination, pp.163-166. James Madison University, Harrisonburg, VA. ”””””””