Connection between rod bivalents and incomplete meiotic association at NORs in Hordeum marinum Huds.

The hypothesis of a connection between rod bivalent formation and incomplete meiotic association at NORs of SAT‐chromosomes of H. marinum is supported. PMCs of H. marinum ssp. gussoneanum (2x), two diploid ssp. marinum × ssp. gussoneanum (2x) hybrids and two ssp. gussoneanum (4x) × Secale cereale hybrids at metaphase I (M‐I) were analyzed by in situ hybridization. The probe pTa71 labelled rDNA sites at NORs of a single pair of homologous or near‐homologous SAT‐chromosomes of H. marinum in each material. In the three diploids, M‐I was regular with ring bivalents and one or a few rods (av.13.52 bound arms cell −1 ). More rod bivalents than the expected one out of seven, i.e. 30, 67 and 89% included rDNA‐carrying chromosomes. Corresponding bound short arm frequencies were 0.89, 0.72 and 0.52, while long arms and arms of other chromosomes presented complete or near‐ complete association. The two heterogenomic hybrids had a less regular M‐I (av. 8.04 bound arms cell −1 ) including 20% rDNA‐carrying rods with bound arm frequencies of 0.29 in short and 0.87 in long arms. Positions of chromosome associations were established in all 150 rDNA‐carrying bivalents. In 77 bivalents with short arm associations, 4% of these occurred proximally to, none at, and 96% distally to rDNA sites, i.e. in satellites. In 143 bivalents with long arm associations, 83% occurred at interstitial and 17% at terminal positions. The observations combine increased frequency of rDNA‐carrying rods with decreased frequency of association at NOR regions of SAT‐chromosomes. The basis for the relationship is discussed.