EM analysis of meiotic chromosome pairing in a pentaploid Achillea hybrid

Abstract

Presynaptic alignment occurs at zygotene and early pachytene in a pentaploid hybrid between Achillea collina (4x) and A. millefolium (6x). Mostly, the five corresponding chromosomes of each set are assorted into a group of three (two synapsed plus one aligned) and a group of two (synapsed); groups of five axes by alignment and/or pairing are extremely rare. This reflects different degrees of homology between the five genomes and allows inferences about the possible phylogeny and genome formulas of the tetraploid (AASS) and hexaploid (AASSTF) involved. The alignment as subgroups of three and two chromosomes in their 5x hybrid also indicates that synaptic alignment responds to slightly different degrees of homology and is decisive regarding which chromosomes will finally pair effectively and cross over. At early zygotene, alignment is weakly expressed, arguing against premeiotic associations as a precondition for homologous alignment and subsequent synaptonemal complex formation. At late pachytene, synapsis is maximized by autosynapsis of chromosomes which do not take part in the preceding homologous two-by-two synapsis.

Inheritance of sesquiterpene lactone types within the Achillea millefolium complex (Compositae)*

“…Referring to former results (Schneider 1958, Ehrendorfer 1963, Loidl et al 1990) we assume that the parent plants are allotetraploid and that chromosome pairing occurs exclusively between the homologous chromosomes of the same genome {AA BB).…”

What makes homologous chromosomes find each other in meiosis? A review and an hypothesis

“…In triploids and higher uneven polyploids the numbers of multivalents are greater than in the corresponding tetraploids, as is the number of partner switches, although theoretically they should be the same (Mather 1939;Loidl 1986;Loidl and Jones 1986;Loidl et al 1990;Vincent and Jones 1993;Jones 1994). Apparently, in triploids, after all three chromosomes have initially attracted each other mutually and two have synapsed at one site, one is left alone, but where no SC has yet formed all three continue to attract each other.…”

Cytogenetics in Plant Breeding

“…This is not an entirely realistic assumption, as is clear from synaptonemal complex studies where parallel alignment (Loidl and Jones 1986;Loidl et al 1990) and several points of pairing partner exchange have been observed repeatedly in several species and hybrids. This is not an entirely realistic assumption, as is clear from synaptonemal complex studies where parallel alignment (Loidl and Jones 1986;Loidl et al 1990) and several points of pairing partner exchange have been observed repeatedly in several species and hybrids.…”