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Discussion
Hybrids and backcrosses production
Hybrids pr oduced 34 % vi able pol lens on a verage, a va lue s imilar t o t hat f ound i n ot her
studies ( 24-28% i n [ 11]; 32 -35% i n [ 14]), t hus a llowing s elf-fertilization t o pr oduce F 2
generation, and possibly serving for outcrossing. However, most of the viable pollen in the
pollen c loud i n a m ixed s tand of B. napus and B. j uncea in th e fields comes from th e two
parent s pecies, s o t hat self-pollination s hould not be t he m ost f requent pol lination e vent.
Hybrids F 1 receiving p ollens f rom B. napus produced t wice as m any BC1 seeds a s t hose
receiving B. juncea pollens. It is likely that pollens of B. napus possessed stronger fertilizing
ability than B. juncea ones [20]. However, the rate of seed set observed here is much lower
than that observed in a similar study, which indicates high variability according to genotypes
and/or conditions [17].
BC1 produced with B. napus (BC1N) had larger seeds, and germinated better than the F2
and BC1 produced with B. juncea (BC1J). These characteristics, together with a b etter ability
to pr oduce B C1 with B. napus , c ould be us eful i n pr edicting a greater pr evalence of t hese
plants, especially at the places where abundant pollens of B. napus occur, i.e. close to oilseed
rape fields. The d ifference o f DNA co ntent b etween t he t wo t ypes o f BC1 suggests a r apid
differentiation within one generation, by around 2 chromosomes on a verage, but there was a
large v ariability w ithin e ach group. C ytological a nalysis o f s imilar p lant ma terial in dicated
that B-chromosomes i n the pr ogeny of h ybrids t end t o be e liminated m ore r apidly t han C -
chromosomes [21], which again suggest a more rapid return to B. napus-like plants.
The expected Mendelian ratios obtained for the resistant phenotype show that this trait is
easy to be transferred between the two species and stably inherited in subsequent generations
of h ybrids w hichever genitor i s us ed a s f emale. S uch a f eature i s not unc ommon i n t he
transmission of transgenes although they h ave n o DNA counterpart in t he recipient species
[10, 12, 22], but distorted segregation was sometimes encountered in different lines [10] or for
different transgenes [23]. In our study, the precise location on t he A, B or C genome of the
ALS ge ne c onferring r esistance ge ne w as not know n, but non -biased s egregation would
suggest an A -chromosome. T his s ituation g ives r ise t o t he p ersistence of t he he rbicide
resistance trait in the field because the hybrid gametes always display native or recombined
A-chromosomes, which would be not the case if it were on a B or C chromosome.
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