- 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
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.