Hybridization between Arctostaphylos viscida and A. canescens in Oregon Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/44558h10b

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  • Arctostaphylos viscida Parry and A. canescens Eastw., two well-known and frequently encountered manzanitas in the Siskiyou Mountains in southwestern Oregon, are shown to hybridize and to produce an extensive series of morphological intermediates. The hybridization occurs wherever populations of these two species meet at contact zones of serpentine and non-serpentine soils. At these locations, A. viscida grows primarily on the serpentine, whereas A. canescens has never been observed on serpentine soils. Chromosome counts made of the two species show that they both have n=13, the haploid number in the genus. Evidence regarding hybridization was obtained through morphological comparisons and the use of the hybrid index and pictorialized scatter diagram techniques. From their morphology, the hybrids are judged to include F₁'s, F₂'s and backcrossed individuals. Most of the identifiable hybrids occur on soils of transitional and non-serpentine types whereas few occur on serpentine soil. This restriction in distribution is believed to be caused by more rigid edaphic selection on serpentine. The effect of differential selection on the survival of hybrid plants is to permit genes of A. viscida to infiltrate more easily the genotype of A. canescens than vice versa. Away from the areas of contact with serpentine soil separate populations of A. canescens are shown to be morphologically variable in the direction of A. viscida. The variability is inversely proportional to the geographical distance separating them from A. viscida populations. The morphological intermediates between A. viscida and A. canescens are similar to six putative species of Arcto-staphylos that were described by Thomas Howell in 1901 from specimens collected in this region of Oregon. These species--A. bracteata, A. strigosa, A. oblongifolia, A. parvifolia, A. cinerea and A. puichella--have been maintained as valid species by several previous authors, even though they are known only from a few herbarium specimens and are extremely difficult to separate taxonomically either from each other or from A. viscida or A. canescens. Since these six putative species can be readily accounted for by hybridization, and since they do not exist as distinctive natural populations but only occur where populations of A. viscida and A. canescens come together, it is suggested that they no longer be given taxonomic recognition.
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