Adaptive nature of chromosomal rearrangement: differential fitness in pocket gophers

A chromosomal centric fusion polymorphism in populations of the plains pocket gopher, Geomys bursarius, was studied to determine the relative fitness associated with the karyotypic phenotypes. There was a greater number of heterozygous individuals than expected χ₁ ²=8.58, P=0.001. Calculations indicate that the viabilities of the two chromosomal homozygotes were only 35 and 76 percent or that of the heterozygote. Differences in fitness values for the chromosomal morphs for Geomys strongly emphasize the possible adaptive nature of the karyotype and provides a primary mechanism for chromosomal evolution, even in species composed of demes of relatively large size. This is the first case of positive chromosomal heterosis in vertebrates. The plains pocket gopher can now be added to the few empirically documented samples of balanced polymorphism.     

PEG-mediated plastid transformation in higher plants

Summary Plastids are surrounded by an envelope consisting of a double membrane. This barrier has to be penetrated or overcome by the DNA when transforming the plastome. Both the biolistic and polyethylene glycol-mediated transformation techniques accomplish this task, albeit by different mechanisms. We were the first laboratory to successfully use the polyethylene glycol (PEG)-method for plastid transformation, yet we use the particle gun when appropriate. In this report we compare the two methods and discuss their shortcomings and advantages. Plastid transformations with various constructs, mainly using theaadA gene as a selective marker, were performed. We point to potential problems likely to be encountered during the transformation and selection processes and offer possibilities for improvement. We give further examples of the successful application of plastome transformation and show its merits in addressing biological questions concerning the elucidation of plastid sequences of unknown function and the control of plastid gene expression. Based on a presentation in the symposium “Organelle Transformation” during the 1997 SIVB Congress held in Washington, DC June 14–18, 1997. An erratum to this article is available at .