An assessment of preferential chromosome pairing at meiosis in Dactylis glomerata

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Abstract
Meiosis was analyzed in 26 full-sib genotypes of tetraploid orchard grass (D. glomerata L., 2n = 4x = 28) that had given apparently nontetrasomic segregation for reaction to the stem rust fungus, Puccinia graminis Pers. f. sp. dactylidis Guyot et Massenot. The mean chromosome associations were 0.24 I, 1.64 rod II, 6.47 ring II, 0.05 III, 0.85 chain IV, and 1.99 ring IV. Optimizations for 2 classes of mathematical models showed an appreciable excess of bivalents over the number expected upon independent association of long and short chromosome arms. The most probable cause of this bivalent excess was preferential pairing due to slight genomic differentiation in an autotetraploid background. Excess bivalents may have also arisen from close proximity of the 2 ends of each chromosome upon initiation of pairing. Inequality of chiasma formation between long and short chromosome arms was a minor contributor of excess bivalents. Because of independent segregation of homologous groups, some sibs fit the Kimber-Alonso 2:2 model best, others the 2:1:1.