Expert: John Locke - 5/3/2007

Question
How do sperms carrying y chromosomes live if x is essential for life?
And what determines which cells in gametophyte(in ferns) turn into antheridia and others into archegonia?
Thanx a lot.

Answer
Sara, thank you for using AllExperts and I'll be happy to answer your questions.

Your first question is an excellent one. Sperm survive because, essentially, they do not use their genetic material as do other cells. Human spermatozoa move by flagella that contain mitochondria; these are bound inside a sheath behind the head of the sperm and, by their production of ATP, provide the energy necessary for contractile fibers in the tail. The contraction of these fibers results in a whipping motion of the tail that propels the sperm forward. No signaling or protein production is required for any part of this process, and as such the DNA in the sperm's head (be it X or Y chromosome) is not used in any way. That's how the Y-carrying sperm survive: they don't need to use their genetic material in any fashion, and thus can survive without a complete genetic complement. As a result of this, the sperm cannot react to its environment in any complex fashion, even to change its direction of travel; sperm thus travel in straight lines at all times.

As to your second question, the determination is not a genetic one, but an epigenetic (environmental) one. The effect of fern size, plant hormones, sunlight, temperature, and nutrition affects how many antheridia vs. archegonia are formed. Of those, plant size and hormones have the most effect; they skew the average distribution of male vs. female gametes away from the 50/50 split that we might normally expect.