Evolution of Plant Size in the Common Morning Glory, Ipomoea purpurea Rick E. Miller Southeastern Louisiana Univ and Mark D. Rausher Duke University Ipomoea purpurea source population in soybean field in North Carolina

Plant size as an ecological trait -- arguably most important trait influencing population biology of plants Universally correlated with fecundity Influences survival Determinant in intra- and interspecific competition Susceptibility to natural enemies -- apparency Ability to tolerate stressful environments Insect herbivore damage on Ipomoea purpurea Photograph by John Stinchcombe

Challenging -- size often differs among individuals due to environmental influences Phenotypic plasticity Ipomoea violacea grown from seed -- same accession Difference in pot size reflects availability of resources small pot larger pot Investigated the evolution of plant size

Research Objectives Additive genetic variation for size? Additive genetic variation for fitness? Pattern of selection acting on size? Genetic correlations between traits –tradeoffs/constraints

Established experimental population of the common morning glory, Ipomoea purpurea, obtaining measures of plant size and fitness

Main results Genetic variation for seed mass and cotyledon size Genetic variation for male fitness (flower production and survival) Positive directional selection acting on seed mass and cotyledon area

Ipomoea purpurea twining vine, common agricultural weed in s.e. US Collected seeds from a large source population in North Carolina soybean field

Sires (males) Dams (females) Offspring Created experimental population using half-sib breeding design 90 sires 270 dams 12 offspring/dam 3,240 seeds

Planted seeds in ploughed field and allowed plants to twine up 2m stakes

Chris Nacci counting flowers Measured size throughout the growing season, including initial mass of each seed. Counted flowers and collected seeds, as well as monitored survival.

Pattern of survival through the growing season for the experimental population of Ipomoea purpurea

Analysis of genetic variation for measures of plant size. Used likelihood- based statistics with explicit spatial models of the fine-scale environmental heterogeneity within the experiment. Likelihoods Trait Sire varianceFull modelWithout sireΩP/2 Initial seed < mass Cotyledon < area Leaf area in August Leaf area in September Leaf area in October Maternal seed mass

seed production

Analysis of genetic variation for reproduction and fitness. Likelihoods Trait Sire varianceFull modelWithout sireΩP/2 Flower production Male fitness Seed production Female fitness

CotyledonLeaf areaLeaf areaLeaf areaFlowerSeedMaternal areain Augustin Septin Octproductionproductionseed mass Initial seed mass P < P < P < 0.01P < n.s.n.s. Cotyledon area P < P < 0.001P < n.s.n.s. Leaf area in August P < P < P < n.s Leaf area in Sept P < n.s.n.s. Leaf area in Oct --n.s.n.s. Flower production ---- Seed0.06 production n.s. Genetic correlations among measures of size are positive and significant. No evidence of a seed size versus seed number tradeoff. Values are based on Best Linear Unbiased Predictors (BLUPs) from models used to estimate variance components.

Pattern of selection acting on initial seed mass involving male fitness (flower production and survival) Genetical analysis of selection using BLUPs ≈ family means.

Ipomoea pedicellaris Oaxaca, Mexico Large seeds Large plants Lots of flowers Genetical analysis of selection Explicit models of fine- scale spatial heterogeneity

Frequency distribution of estimated linear selection gradients for quantitative traits in natural populations, compiled from 63 studies. Hoekstra et al PNAS 98: Positive directional selection Initial seed mass β = Cotyledon area β =

Is this population of Ipomoea purpurea evolving in the direction of larger seeds?

John Harper (1977) in Population Biology of Plants provided comprehensive review of demographic studies. Emphasized special attention be paid to the early fate of individuals in life cycle of plants, especially for annual species. This study supports this view -- finding significant selection acting on initial seed mass and cotyledon size, but not on size later in the growing season

Acknowledgements Chris Nacci –fantastic research assistant National Science Foundation Louisiana Board of Regents