Graduate Thesis Or Dissertation

Biosystematics of the Mimulus nanus complex in Oregon

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  • A biosystematic study was made in seven populations of Mimulus nanus Hook. & Arn. and M. cusickii (Greene) Piper (Scrophulariaceae) in central Oregon, and a taxonomic revision was made of the four species of section Eunanus reported from Oregon--M. nanus, M. cusickii, M. clivicola Greenm. and M. jepsonii Grant. Mimulus nanus and M. cusickii have a chromosome number of n = 8. Based on their distinct genetic and morphological differences, M. nanus, M. cusickii and M. clivicola constitute three separate species in Oregon and surrounding regions. Members of M. nanus are the most highly variable in their morphology and are more widely distributed geographically and ecologically. In a limited area of the Cascade Mountains of central and southern Oregon, an ecotype of M. nanus was discovered which differs from the typical form that is widely distributed in Oregon and Idaho. Also, the populations that have previously been named M. jepsonii, occurring in the southern Cascade and northern Sierra Nevada mountains, Oregon and California, are herein treated as an ecotype of M. nanus; they are morphologically similar to this taxon but show differences in ecology and elevational range. The two ecotypes mentioned above appear to hybridize with typical M. nanus at their zones of contact, thus demonstrating the ability for genetic exchange in nature. Cross-compatibility was confirmed in greenhouse hybridizations between the Cascade ecotype and typical M. nanus. Therefore, based on ecogeographical and morphological criteria, M. nanus is segregated into three subspecies: M. nanus Hook. & Arn, ssp. nanus, M. nanus ssp. cascadensis Ezell and M. nanus ssp. jepsonii (Grant) Ezell to include the form previously named M. jepsonii. Greenhouse hybridization studies show that M. cusickii and M. nanus ssp. are genetically isolated for the most part; however, on the basis of seed-set data, there is the potential for genetic exchange between M. cusickii and M. nanus ssp. cascadensis. In sympatric field populations of M. cusickii and M. nanus, no hybrids were detected, thus implying that other isolating mechanisms, i. e. , temporal factors or pollinator preference, may exist. No consistent correlation is found between capsule length and plant height, and capsule length and the number of seeds per capsule in any of the experimental populations of M. nanus ssp. nanus, M. nanus ssp. cascadensis and M. cusickii. These data suggest that capsule length, plant height and seed-set vary independently of one another, which implies that they are not developmentally linked. Statistical analyses of capsule lengths and days from pollination to capsule maturation reveal a greater degree of variability in M. nanus ssp. cascadensis than in the other two taxa. This variability suggests, in part, that populations of subspecies cascadensis may be less buffered against environmental effects on development, or may be of more recent origin through hybridization or introgression, which would have to lead to greater genetic variability.
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