One gene determines maize B chromosome accumulation by preferential fertilisation; another gene(s) determines their meiotic loss.

Genotypes of high (H(m)) and low (L(m)) male B transmission rate (B-TR) were obtained. B-TR segregation in the F2 is reported, showing that the H(m) and L(m) lines differ in a single locus we call mBt (male B transmission), controlling B preferential fertilisation in maize. The egg cells control which one of the sperm nuclei is going to fertilise them, mBt(h) egg cells being preferentially fertilised by the sperm nucleus carrying the supernumerary B chromosomes (Bs). It is hypothesised that the mBt gene is involved in the normal fertilisation of maize but the parasitic Bs take advantage of the mBt(h) allele to increase their own transmission. Selection was also carried out when the Bs were transmitted on the female side (H(f) and L(f) lines). The F1 hybrids show that the gene(s) that we call fBt (female B transmission), controlling female B-TR, is located on the A chromosomes acting at diploid level, the fBt(l) allele(s) for low transmission being dominant. This allele causes the loss of Bs at meiosis, which is shown using a specific B molecular probe to determine B presence/absence in microspores of both lines and hybrids. Maize Bs are a nice example of intragenome conflict, because the mBt and fBt loci are a polymorphic system of attack and defence between A and B chromosomes.