Chromosome pairing affinity and quadrivalent formation in polyploids: do segmental allopolyploids exist?

Abstract

When polyploid hybrids with closely related genomes are propagated by selfing or sib-breeding, the meiotic behaviour will turn into essentially autopolyploid behaviour as soon as the affinity between the genomes is sufficient to permit occasional homoeologous pairing. An allopolyploid will only be formed when the initial differentiation is sufficient to completely prevent homoeologous pairing (in some cases enhanced by specific genes), or when segregational dysgenesis prevents transmission of recombined chromosomes. A new polyploid hybrid may be considered a segmental allopolyploid and may show reduced multivalent formation as a result of preferential pairing between the least differentiated genomes. An established polyploid is either an autopolyploid or an allopolyploid. In exceptional cases it is thinkable that a stable segmental allopolyploid arises, in which some sets of chromosomes are well differentiated and behave as in an allopolyploid, whereas other sets are not well differentiated and behave as in an autopolyploid. No clear cases have been found in the literature so far. Key words : chromosome, pairing affinity, quadrivalent frequency, segmental allopolyploidy.

Chromosome numbers and meiotic behavior of some Paspalum accessions

“…The low frequency of quadrivalents in tetraploid accessions reported in several Paspalum species has been interpreted as resulting from segmental allopolyploidy (Burson and Bennett, 1971;Norrmann et al, 1989;Quarín et al, 1996;Takayama et al, 1998;Pagliarini et al, 2001). Although low multivalent frequency is an argument frequently used in advocating segmental allopolyploidy, Sybenga (1996a) pointed out that this character is not necessarily a reliable indication of limited pairing affinity, and thus of homology, because even true autopolyploids may form quadrivalents with frequencies substantially lower than theoretically possible. Quarín (1992) has proposed that at least in apomictic Panicoid grasses, polyploidy is a condition for apomixis and that apomixis is associated with autoploidy rather than alloploidy.…”

Extensive chromosomal variation in a recently formed natural allopolyploid species,Tragopogon miscellus(Asteraceae)

“…He proposed that residual homology between homeologous chromosomes is primarily responsible for inconsistent bivalent formation and nondisomic inheritance. Meiotic irregularities may render segmental allopolyploids unstable (58,59), with increasingly rearranged karyotypes and shifts to polysomic inheritance (59). Stebbins's definition was based on a concept of structural homology between progenitor chromosomes, but its application was based on patterns of inheritance and/or chromosome behavior, with an underlying assumption of additivity of the parental genomes.…”

Estimation of allele frequencies in polyploids under certain patterns of inheritance

“…There are several recent papers on this subject, both from a cytogenetic and a mathematical modelling point of view (Sybenga, 1994(Sybenga, , 1995(Sybenga, , 1996(Sybenga, , 1999Jackson and Jackson, 1996). The evidence suggests that these situations should be characterised by a general polyploid inheritance model with no complete preference to homologous or homeologous paring (Wu et al, 2001).…”