Origin of a polyploid Botrytis pathogen through interspecific hybridization between Botrytis aclada and B. byssoidea

Botrytis aclada has previously been divided into two subgroups (AI and All) based on spore size, chromosome number and genetic markers. A recent study of genetic diversity using universal-primed PCR (UP-PCR) fingerprints showed that subgroup All of B. aclada was a possible hybrid species between B. byssoidea and subgroup Al of B. aclada. To test this hypothesis, we sequenced a UP-PCR fragment (-550 bp) from representative isolates of four Botrytis spe- cies, all causing disease on onions. A mixture of two nucleotides at each nucleotide position was found in all All isolates analyzed at 23 positions in the se- quence. These isolates had 32 chromosomes, whereas all Al isolates had 16 chromosomes. Pair-wise com- parison of the sequences of the four Botrytis species showed that a hybridization event between B. aclada (AI) and B. byssoidea could explain all 23 positions with mixed nucleotides, supporting them as being parental ancestors to All. These results indicate that at least one Botrytis pathogen has evolved through interspecific hybridization.