Polyploid Classification:
Autopolyploids: result from an increase in ploidy level within a species (AAAA)
Allopolyploids: Result from hybridization between species
- Genomic allopolyploids: carry entire chromosome sets from 2 or more species. Chromosomes from different species do not pair during meiosis. (AABB)
- Segmental allopolyploids: some chromosomes do not pair, others do. Often unstable and continue to evolve. (A1A1A2A2)
- Autoallopolyploids: involve both autopolyploidy and allopolyploidy (AAAABBBB)
Pathways to polyploidy: More than just these 3, but this is a good start.
- Somatic doubling: Mitotic products in a diploid cell fail to segregate to opposite poles
- Meiotic nonreduction: cell wall fails to form late in meiosis, yielding diploid gametes.
- Polyspermy: 2 sperm fertilize a single egg -> (intermediate) triploid
Incidence: Polyploidy occur(s/ed) in ferns, moss, algae, virtually all groups of vascular plants, yeast, ancient vertebrates (why verts have 4x more chromosomes than inverts), ancient fish. Polyploidy is the norm in ferns, common in angiosperms, but rare in gymnosperms. Frequency varies across groups.
Frequency of auto vs. allopolyploidy: arise in nature at rates equivalent to genetic mutation. Allopolyploids considered more common in past literature. However, autopolyploids are probably more common than once thought; they’re just hard to identify. Allopolyploids still probably more common.
Ecology and persistence: polyploids are thrown into immediate competition with their diploid ancestors. Most probably go extinct quickly. Assortive mating and ecological differences required for a stable population of polyploids.
Why is polyploidy rarer in animals than in plants?
Nobody really knows, but there are some hypotheses. Author favors own hypothesis: transition from diploidy to tetraploidy disrupts dosage compensation of X chromosome(s). Hypothesis hinges on possession of smaller sex chromosome for heterogametic sex. – Consequences of commitment to chromosomal sex determination. However, author states that so little data exists that nobody should be surprised if all current hypotheses turn out to be wrong.
