Hierarchical control of multivalent formation in natural populations of a hexaploid perennial grass ( Koeleria vallesiana )

Abstract

Observations on variation in multivalent frequency in Koeleria vallesiana are reported. The species is self-incompatible and has a standard karyotype divided into six haploid genomes each of four metacentric and three submetacentric euchromatic chromosomes (2.6–3.6 μm), one of the submetacentrics being a nucleolar–organizer with a minute satellite on the short arm. Of the 83 plants examined, 67 had the hexaploid number 2 n = 6 x = 42,12 were aneuploids (2 n = 41-45) and four had a single B-chromosome. At first metaphase of meiosis 92% of chromosomes form bivalents. Quadrivalents are the most common multivalents (5.5%) and sexivalents (1.1 %), the largest. Each chromosome of the standard complement is equally likely to be incorporated in a multivalent. There is extensive variation in chiasma number per pollen mother cell between cells (21–43) and plants (22.6–38.8) but not between populations (27.6–31.5) or countries (29.1–30.1). Variation in multivalent frequency is partly chiasma-dependent and partly synaptic in origin. It is not associated with the variation in chromosome number. Phenotypic variation at synapsis influences the frequency of pairing partner exchange and is found both between plants and between populations. Some populations are more variable in synaptic behaviour than others. The mechanisms determining such synaptic variation are unknown but their investigation should provide a useful insight into the general nature of chromosome homology as well as shedding light on evolutionary processes in polyploids.