Karyotype Variants B: Chromosome Number Variants

Abstract

The term haploid is used as an adjective (referring to a specific number of genomes of chromosomes of an individual) and as a noun (referring to the individual itself). In both cases it can have two meanings that often overlap, but that in some situations may cause confusion. Haploid is used: (1)for the chromosome number of the gamete (the haplont); (2) for the chromosome number of a single genome (in diploids) or of a set combined genomes segregating together (in allopolyploids). In diploids and allopolyploids the two meanings coincide. In autopolyploids they are different: the gametic chromosome number is half the polyploidy number and, consequently, a multiple of the basic genome number. For this reason the basic chromosome number is often referred to as monohaploid or monoploid. Linguistically, these terms are far from elegant, as is not unusual in biological nomenclature. In allopolyploids and less, appropriately in autopolyploids, the gametic chromosome number is also referred to as dihaplod, trihaploid, etc., in general: polyhaploid, If an allopolyploid is reconstructed by doubling the chromosome number of the hybrid between the parental species, this hybrid has the same chromosome number, but is not normally referred to as a (poly)haploid. Somatic reversion of a tetraploid number to the diploid number (Gottschalk 1976) would not be a reason to use the term haploid for the plant originating from it. Also, for all practical purposes, a plant derived from an unreduced gamete of an autotetraploid is a diploid, and this is the preferred term in this case. For an individual plant, the principal criterion for deciding whether the ploidy level is haploid or polyhaploid is the number of genomes rather than its origin.