Sexual polyploidization and depolyploidization: some terminology and definitions.

Sexual polyploidization (deployploidization) is the process through which a euploid zygote is formed whose chromosome number is increased above (decreased below) the level that would be expected if each parent contributed a gamete carrying one-half the parental premeiotic chromosome number. One or both gametes may contribute (lack) the extra genome (s) so that these processes may be unilateral or bilateral. Sexual polyploidization has the distinctive characteristic of being a genetically innovative mechanism. This is believed to be its most significant difference with respect to somatic doubling. Sexual depolyploidization, which may be conceived to arise in some rare situations, also has the capacity for innovation. Several cytological mechanisms may be envisioned which give rise to polyploidizing gametes. The genetic consequences of these mechanisms can be markedly different in regards to gene reassortment. Short of a mitotic-like method, FDR (first division restitution) appears to be the most appropriate mechanism to attain such a goal. Other modified meiotic processes generally result in more disturbance of the premeiotic genotypes. Under these circumstances, sexual polyploidization is probably associated with mitigated heterotic responses and a more extensive release of genetic variability.As a cytological device, FDR has the unique property of producing balanced 2n gametes even after a numerical nondisjunction has occurred. This simple mechanism may restore fertility to, e.g., monoploids and triploids, so that odd-ploids may possess more evolutionary significance by acquiring a means of producing balanced gametes systematically.Some new terminology is proposed to designate gametes and gametophytes which conserve the parental number of chromosomes, and for polyploids arising in conjunction with their functioning in fertilization.