GENETIC ANALYSIS OF STANDING VARIATION FOR FLORAL MORPHOLOGY
AND FERTILTY COMPONENTS IN A NATURAL POPULATION OF ANNUAL
MIMULUS GUTTATUS (COMMON MONKEYFLOWER)
Young Wha Lee*, John H. Willis*, John K. Kelly#
*UPGG/Dept. of Biology, Duke University; #Dept. of Ecology and Evolutionary
Biology, University of Kansas
An unresolved problem in evolutionary biology is the nature of forces that
maintain standing variation for quantitative traits. In this study we take advantage of
newly developed genomic resources to understand how variation is maintained for flower
size in a natural population of annual Mimulus guttatus in the Oregon Cascades. Flower
size is an ecologically functional trait implicated in attraction of pollinators, water use
efficiency, age at anthesis, and reproductive capacity. Previous biometric experiments
have demonstrated that some of the floral variation in this site is due to common alleles
perhaps maintained by balancing selection.
We present here the first results from a large scale effort to genetically dissect
variation for flower size on Iron Mountain. First, in 3 independent F2 mapping
populations we mapped QTLs for floral morphology (flower width and length, pistil
length, and stamen length) and fertility components (pollen viability and seed set).
Flower size is a polygenic trait: QTLs are small to moderate in magnitude and there is
hardly any co-localization between crosses in mapped QTLs. There is little pleiotropy
between floral morphology and fertility traits, suggesting that variation in flower
morphology is not largely a by-product of unconditionally deleterious alleles. Degree of
pleiotropy between different components of floral morphology varies, an interesting
observation given the range of floral allometries associated with mating system evolution
in the guttatus species complex. Secondly, we have created 200 near-isogenic lines
(NILs) carrying flower size QTLs with 2 of the 3 F1 individuals used to generate the F2
mapping populations as donor parents. We applied a breeding scheme that incorporated
QTL mapping for flower width with NIL generation by means of phenotypic selection to
enrich (above a null expectation threshold) the final set of NILs for flower size QTL.
Many of the same flower size QTLs are mapped in the NIL populations as in the F2,
indicating that these QTLs are robust to effects of genetic background as well as the
variable environmental conditions encountered through five phenotyping growouts in two
different greenhouses. These QTLs are good candidates for fine-mapping and positional
cloning. The NILs, along with an existing set of 200 inbred lines randomly extracted
from Iron Mountain, provide the genetic materials for defining the alleles underlying
QTLs through a combination of recombinational and LD mapping. The identification of
the segregating alleles for flower size on Iron Mountain will allow us to directly address
the importance of selection in maintaining quantitative trait variation.