It is a fact that autotriploids have a hard time establishin

It is a fact that autotriploids have a hard time establishing themselves in nature when they arise. a) Why would this be the case? b) If autotriploids are in fact highly sterile, how is it possible to maintain varieties like the Keizerskroon tulip and the Baldwin apple, which are known to be autotriploids? c) Describe how autotriploidy could arise in each of the following cases: cross-fertilization, self-fertilization, asexual reproduction. d) In humans, approximately 1% of ALL zygotes produced are autotriploids or autotetraploids, but such ploidy is inviablc (Kajii, T., and N. Niikawa, Cytog. Cell Genet. 18: 109, 1977; Pettenati, M. J., et al., Am. J. Hum. Genet. 24: 23, 1986). How might autotriploidy arise in such a zygote?

Solution

a) Autotriploids have a hard time establishing themselves in nature when they arise as they are highly sterile. This is due to disturbed alignment of homologous chromosomes in prophase I followed by altered segregation in anaphase I which in turn leads to uneven distribution of gametes.
b) The autotriploid varieties like Keizerskroon tulip and Baldwin apple are maintained by vegetative propagation (clonal propagation) rather than sexual reproduction.
c) CROSS FERTILIZATION: Autotriploidy arises by fusion of diploid and haploid gametes during sexual reproduction. The diploid gamete can come from autotetraploid or nondisjunction of chromosomes during anaphase in gametogenesis in a diploid parent.
SELF FERTILIZATION: Non disjunction of chromosomes during megasporogenesis produces diploid gametes which then fuses with haploid gamete on self fertilization and give rise to autotriploid.
ASXUAL REPRODUCTION: Consecutive DNA replication during S phase followed by arrest of cell division leads to formation of autotriploids during asexual reproduction.
d) Fusion of diploid egg with haploid sperms forms autotriploid zygotes in human.