A MEIOTICALLY REPRODUCIBLE CHROMOSOME LENGTH POLYMORPHISM IN THE ASCOMYCETE FUNGUS OPHIOSTOMA-ULMI (SENSU-LATO)

We have followed the transmission of Ophiostoma ulmi s.l. chromosome l ength polymorphisms (CLPs) into the F-2 generation to determine the re producibility of a genome rearrangement culminating in the conversion of a 1.0 Mb chromosome into a 800 kb chromosome. The 1.0 Mb chromosome in strain CESS16K is thus far unique among O. ulmi s.l. wild-type str ains, as no other wild-type strains have been observed with chromosome s smaller than 2.3 Mb. It has been previously shown that the 1.0 Mb ch romosome is mitotically stable, carries at least one normally expresse d gene, and is transmitted through meiosis. In this study, a series of crosses were performed to further elucidate the pattern of inheritanc e of the 1.0 Mb chromosome and the process of conversion of the 1.0 Mb species to 800 kb. In crosses where the 1.0 Mb chromosome was allowed to pair with itself or with the 800 kb chromosome, all progeny inheri ted a copy of the 1.0 Mb or 800 kb form, further demonstrating the A-t ype nature of these small chromosomes. When a cross was repeated betwe en the strains CESS16K (1.0 Mb chromosome) and FG245B(r)-O (no 1.0 Mb or 800 kb chromosome)? the occurrence of a 800 kb chromosome was obser ved in 9% of the progeny. A reciprocal cross between an 800 kb strain and a strain with no 800 kb or 1.0 Mb chromosome was conducted, and a progeny strain containing a 1.0 Mb chromosome was recovered. The repro ducibility and reciprocality of the 1.0 Mb to 800 kb chromosome conver sion demonstrates that meiotic processes are responsible for this CLP, and that O. ulmi s.l. strains with various divergent genome architect ures can remain sexually compatible.