Genetic Variation In a Biocontrol Target Weed: The Strawberry Guava Species Complex

The leaf-galling scale insect, Tectococcus ovatus, has been proposed for introduction in Hawaii as a host-specific biocontrol agent to limit invasive strawberry guava (SG) populations in mid-elevation native forests. Our objective is to characterize the genetic variation in SG populations in Hawaii, with the goal to inform the biological control efforts currently being developed. Specimens collected on the islands of Hawaii, Maui, and Oahu were evaluated for fruit and vegetative morphology, ploidy as determined by flow cytometry, and microsatellite variation at four chloroplast SSR loci and four nuclear SSR loci. Results supported three previously recognized taxa and several new categories. Psidium littorale was uniform with regard to fruit morphology (yellow, spindle-shaped), ploidy (8x), and SSR polymorphisms, suggesting that it may be a fertile allo-octoploid or a sterile apomict. Similarly, P. lucidum was nearly, but not completely, uniform in the same aspects (yellow, spherical fruits; ~6x ploidy), suggesting a fertile allo-hexaploid or an apomict with some residual sexual function. P. cattleianum displayed a single uniform chloroplast SSR genotype, but numerous nuclear SSR genotypes, ploidy variation between 6.5x and 7x, and red fruit color of variable hue and intensity, suggesting that sexual reproduction is operative in this nominally heptaploid form and that it produces mainly aneuploid progeny. A fourth form (Psidium “X”) with fruit color (orange) and ploidy (7.5x) intermediate between those of P. cattleianum and P. littorale originally suggested a derivation through interspecific sexual crossing.  However, the presence of unique chloroplast and nuclear SSR alleles found in the orange-fruited forms and not in either of the putative parent species indicates that some of these may represent a new taxon not previously described in Hawaii. The broad range of strawberry guava genotypes documented in this survey, compounded by the potential for new variation arising through sexual processes, argues that efficacy of biological control of this species complex in Hawaii can be accurately estimated only after extensive testing of representative genotypes.