Maximizing Heterozygosity in Autopolyploids

Abstract

Polyploidy appears dependent on heterozygosity! The largest group of po1yp1oids, the a11opo1yp1oids (disomic po1yp1oids), have fixed heterozygosity in the two or more divergent genomes they possess (e.g., wheat, oats, cotton, tobacco, etc.). Hexaploid wheat, for example, although self-pollinated and basically homozygous at loci in each of its three genomes, has internal hybridity among loci with similar function in its three genomes. Disomic po1yp1oids thus are able to capitalize on the merits of both the se1f-and cross-fertilizing systems (1).The autopo1yp1oids (polysomic po1yp1oids) insure their heterozygosity through cross-pollination (e.g., alfalfa, birdsfoot trefoil, potato, and many grasses). We can find no example in crop plants of a successful polysomic polyploid species which is self-pollinated. Evidently the polysomic condition can not tolerate the homozygosity associated with self-pollination. The biochemical and physiological advantages of heterozygosity must be an important component of polyploid vigor (2). Polysomic po1yp1oids are represented by segregating and heterogeneous populations where maximum heterosis may be expressed by a few elite individuals in the population but not by the population per se. (Apomixis can preserve and perpetuate the elite polyploid individuals but will not be reviewed here.) In our seed reproduced polysomic po1yp1oids we can maximize the frequency of elite genotypes, but at present we cannot fix such genotypes in cultivated populations. As will be illustrated in this paper, maximum heterozygosity and heterosis does not occur in the Fl or single cross generation when parents are inbred (as it does 1n diploids or disomics) but occurs in the segregating double cross or an even later generation. Under polysomic polyploid conditions, the more inbred the parents, the lower the 471 W. H. Lewis (ed.), Polyploidy

Allozyme polymorphism in diploid and polyploid populations of Galium

“…There is evidence for somewhat reduced activities of multiple copies of enzyme loci in polyploids, but no suggestion of their complete "silencing". These data are in line with theoretical and factual evidence from several polyploid complexes (both auto-as well as allopolyploid) studied in a comparable way (Barber, 1970;Bingham, 1980;Lumaret, 1984).…”

Ploidy Effects in Isogenic Populations of Alfalfa

“…Greenhouse-grown plants were higher than growth chamber plants in BSP per mg DNA (P < 0.05) and RuBPCase activity per mg RuBPCase (P < 0.01), but lower in RuBPCase concentration (P < 0.05) and RuBPCase per mg BSP (P < 0.01). DISCUSSION Polyploid evolution is a complex process that is dependent on chromosome doubling, heterozygosity, mutation, and regularization of chromosome-pairing (3,27). Study of the physiological and biochemical effects of polyploidization as an individual process of genome duplication requires plant material which differs only in chromosome number.…”

“…Although chromosome number probably varied in the 8X population, the expression of data on a DNA basis allowed a correction for this variability. Maximum heterozygosity, or the highest frequency of tetraallelic loci in tetraploids, has been proposed to be a requirement for full fertility and vegetative vigor in alfalfa (3). The frequency of octo-allelism in the octoploid population may have been insufficient for maximum heterosis.…”

Ploidy Effects in Isogenic Populations of Alfalfa

“…Such contradictory observations can result from genetic limitations of the plant material under investigation. Genetic differences between allopolyploid genotypes (4,15) and inbreeding depression associated with colchicine-induced polyploids (2,25) may confound the consequences ofploidy effects. To minimize these limitations in the study of ploidy effects, isogenic diploidtetraploid and tetraploid-octoploid alfalfa (Medicago sativa L.) populations, to which heterozygosity was restored, were developed.…”