Meiotic behaviour of 11 primary trisomics in diploid potato and its consequences for the transmission of the extra chromosome.
Meiosis was studied in 11 primary trisomics (2n = 2x + 1 = 25) from diploid Solanum tuberosum L. ssp. tuberosum Hawkes (2n = 2x = 24) and from interspecific Solanum hybrids. The three homologous chromosomes were associated in a trivalent in 90% of more than 175 pollen mother cells analysed at pachytene. Trivalents showing a two by two pairing and partner exchanges at pachytene along with incomplete triple synapsis were frequently observed. Foldback pairing, predominantly observed in the heterochromatic parts of the chromosomes, occurred in 28.3% of the trivalents. Nonhomologous association of chromosome segments was observed in 29.1% of the trivalents. Up to six telomeres were associated homologously in 46.2% of the trivalents. Genotypic differences with respect to trivalent formation at metaphase I occurred in almost all the trisomic types and was significant for the trisomics for chromosomes 4, 7, and 9. The coefficient of realization of a trivalent (CRT) at metaphase I in the 11 primary trisomics varied from 0.20 to 0.80, and was positively correlated with the absolute length of the extra chromosome (r = 0.61, p &le; 0.05) and with the absolute, as well as the relative, length of the euchromatic segments of the extra chromosome (r = 0.70, p &le; 0.05). There was no apparent relationship between the CRT and the distribution pattern at anaphase I or anaphase II. The rate of female transmission of the extra chromosome varied from 10.0 to 44.6% among different trisomics. For chromosomes 4, 7, and 9, this rate differed significantly among genotypes of the same trisomic.
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