Mitochondrial plasmids in related cultivars of crookneck and stable and unstable butternut squash

Summary

Two series of two to four plasmids are contained in mitochondria of butternut squash (Cucurbita moschatd). The plasmids are composed of DNA, and are consistent in number within a cultivar but vary in number among cultivars. Plasmid patterns and homology suggest that the faster migrating series of plasmids are the supercoiled form of the slower running plasmids. Plasmid 1 was present in all cultivars except New Hampshire Butternut. Plasmid 2 was only observed in the original crookneck cultivar, Canada Crookneck. Plasmids 3 and 4 were present in all accessions tested. The presence of plasmid 1 in a cultivar does not follow a maternal pattern of inheritance; this may be due to the ability of the plasmid to insert into and excise from the main mitochondrial DNA, or the recombination of some of the smaller plasmids to create the larger plasmid 1. There is some homology between plasmid 1 and plasmids 3 and 4, and between plasmid 1 and main band mitochondrial DNA. The equal presence of plasmid 1 in mitochondria of F₁ seedlings from the pollination of New Hampshire Butternut by Ponca Butternut to that in F₁ seedlings of the reciprocal cross indicates that there is probably a dominant nuclear effect on the ability to produce plasmid 1 either by release from the mitochondrial genome or by recombination of the smaller plasmids. There is no obvious relationship between the presence or number of the mitochondrial plasmids and the butternut fruit shape or stability of the butternut trait of the cultivars.