Genome affinity and meiotic behaviour in trigenomic hybrids and their doubled allohexaploids between three cultivated Brassica allotetraploids and Brassica fruticulosa

Introduction Polyploidy has played a major role in the evolution of higher plants, and polyploids have often been selected during the evolution of crop plants (Levin 2002). As now generally [...]
The wild species Brassica fruticulosa Cyr. (FF, 2n = 16) is closely related to the cultivated Brassica species. Through interspecific reciprocal crosses between B. fruticulosa and three cultivated Brassica allotetraploids (AABB, AACC, and BBCC where A = 10, B = 8, and C = 9), four trigenomic hybrids (F.AC, 2n = 27; F.AB, 2n = 26; F.BC, 2n = 25; BC.F, 2n = 25) were produced. By chromosome doubling of respective hybrids, three allohexaploids (FF.AACC, 2n = 54; FF.AABB, 2n = 52; BBCC.FF, 2n = 50) were synthesized. In pollen mother cells (PMCs) of the trigenomic hybrids, 1-2 autosyndetic bivalents were detected within A, B, and C genomes but only one within F genome; 1-3 allosyndetic bivalents between any two genomes were observed, and a closer relationship of F and B genomes than F and A genomes or F and C genomes was revealed. The allohexaploids showed a generally low but different pollen fertilities. The chromosomes in PMCs were predominantly paired as bivalents but some univalents and multivalents at variable frequencies were observed. The bivalents of homologous pairing for each genome prevailed, but allosyndetic quadrivalents and hexavalents involving any two genomes were observed, together with autosyndetic quadrivalents for A, B, and C genomes but not the F genome. The nondiploidized cytological behaviour of these allohexaploids contributed to their low fertility. The relationships between the genome affinity and meiotic behavior in these allohexaploids were discussed. Key words: cultivated Brassica allotetraploids, Brassica fruticulosa, interspecific hybrids, autosyndesis, allosyndesis, diploidization. L'espece sauvage Brassica fruticulosa Cyr. (FF, 2n = 16) est tres apparentee aux especes cultivees du genre Brassica. Suite a des croisements interspecifiques reciproques entre le B. fruticulosa et les trois Brassica allotetraploides (AABB, AACC et BBCC oU A = 10, B = 8 et C = 9), quatre hybrides trigenomiques (F.AC, 2n = 27; F.AB, 2n = 26; F.BC, 2n = 25; BC.F, 2n = 25) ont ete produits. Suite a un doublement chromosomique chez ces hybrides, trois allohexaploides (FF.AACC, 2n = 54; FF.AABB, 2n = 52; BBCC.FF, 2n = 50) ont ete obtenus. Au sein des microsporocytes (PMC) des hybrides trigenomiques, 1-2 bivalents autosyndetiques ont ete detectes au sein des genomes A, B et C, mais un seul a ete observe pour le genome F. Un a trois bivalents allosyndetiques entre n'importe lesquels des genomes ont ete observes, et une plus grande proximite entre les genomes F et B qu'entre les genomes F et A ou F et C a ete constatee. Les allohexaploides affichaient une fertilite du pollen differente mais generalement faible. Les chromosomes dans les microsporocytes etaient principalement apparies sous forme de bivalents, bien que des univalents et des multivalents aient ete observes a des frequences variables. Les bivalents impliquant des chromosomes homologues de chaque genome etaient predominants, bien que des quadri-/hexavalents allosyndetiques impliquant deux genomes aient ete observes, de meme que des quadrivalents autosyndetiques pour les genomes A,B et C, mais pas pour le genome F. Le comportement cytologique non-diploide chez ces allohexaploides a contribue a leur faible fecondite. Les auteurs discutent des relations entre l'affinite des genomes et le comportement meiotique au sein de ces allohexaploides. Mots-cles : Brassica allotetraploides cultives, Brassica fruticulosa, hybrides interspecifiques, autosyndese, allosyndese, diploidisation. [Traduit par la Redaction]