GROSS, BRIANA*, KARI ROLLENHAGEN, JAMIE MELLOR, and SUSAN KEPHART. Department of Biology, Willamette University, Salem, OR 97301. - Species boundaries in Camassia: Analysis of a putative hybrid zone.
In classic evolutionary models, species remain distinct when isolated
by one or more extrinsic or intrinsic barriers to hybridization. If
these barriers reduce or prevent genetic exchange, the morphological
discontinuities among reproductively isolated species will provide an
effective mechanism for their classification. We used morphological
and molecular analyses, and data on flowering phenology, breeding
systems, and pollination, to explore the taxonomic relationships
between two camas lilies, Camassia leichtlinii and C.
quamash, which occur sympatrically in Oregon. Differing
classifications persist for these North American taxa: Gould treated
the species as distinct in his monograph, noting the absence of
hybrids, whereas a recent California flora relegates C.
leichtlinii to a subspecies of C. quamash Our surveys of
single and mixed populations show extensive overlap in extrinsic
factors such as flowering time and insect visitors. Morphological
study confirms the presence of an intermediate form that differs
significantly (P < 0.05) from one or more parental species in
all but one of ten characters measured. Thus, pre-zygotic barriers
appear weak, and interspecific hybridization may occur in
Camassia. Preliminary allozyme analysis of ten loci also
revealed additive banding patterns and higher than average
heterozygosities in putative hybrid zones. Of ISSR primers analyzed to
date, two diagnostic bands of the parental species C.
leichtlinii and C. quamash appear in 78% and 9 % of the
putative hybrids, respectively. Yet hybrid seeds produced from
artificial pollen manipulations were inviable under glasshouse
conditions, suggesting the presence of intrinsic, post-zygotic
barriers to interbreeding. These data and multivariate numerical
analyses imply that C. leichtlinii and C. quamash are
best classified as separate species, but occasional hybridization and
introgression may also play an important role in their evolution.
Key words: Camassia, hybridization, Liliaceae, reproductive isolation, speciation