Testing for asymmetric response to selection for
flowering time in Brassica rapa
Supreet Sunil, Emily Austen, Arthur E. Weis
Department of Ecology and Evolutionary Biology, University of Toronto
BACKGROUND
RESULTS
 Evolutionary shifts in phenological characteristics, such
as flowering time, are important in species’ response to
changing climatic patterns, biological invasions, and other
biotic & abiotic factors.1,2
(A)
(B)
 Previous work indicates that such evolutionary shifts
can occur rapidly due to strong capacity to respond to
selection pressures.2
Recent studies have revealed asymmetric responses to
selection on flowering time, suggesting that some species
may not respond equally to selection for early and late
flowering.3
Fig 2. (A) Variance in offspring bolting time increases with maternal bolting date (n = 315 families, mean 11.6 individuals per family).
(B) Trend towards increasing variation in offspring bolting day with maternal bolting day persists even when variation is meanstandardized, which should correct for scale effects.
 This is consistent with observations in our lab that
variance in offspring flowering date increases with later
maternal flowering, potentially indicating reduced
selection response (Figure 1).
(B)
(A)
Early
Ctrl
Late
Response to selection may also be constrained by
selecting for correlated traits (such as flowering time and
plant size).4
Asymmetric responses to selection may arise due to
several factors, the most important of which are genetic
asymmetry and scale effects.5
2
mean offspring flowering time
1.5
1
Fig 3. (A) Selection response for late flowering was surprisingly strong in all lines (HA = black, NA = grey). (B) Flowering time variation
is greater in late selection line than in early selection line. Distributions shown for HA rep 1.
0.5
0
(A)
-0.5
(B)
RHA=0.72
-1
-1.5
-1.5
RNA =0.66
-1
-0.5
0
0.5
1
1.5
2
2.5
midparent flowering time
Figure 1. Mid-parent regression of maternal flowering date on offspring flowering
date. Note the large variance in offspring flowering for late flowering maternal plants
(C)
HYPOTHESIS
Because parent-offspring resemblance is stronger in
early flowering plants, we expect that a population
will respond more rapidly to selection for early
flowering than for late.
Fig 4. (A) Strong correlation between size at flowering (leaf scars) and
flowering time suggests that selecting on flowering time will cause indirect
selection on size as well.
(B) & (C) Strength of relationship between flowering time and size (LS and SD)
is stronger in HA populations than NA populations.
STUDY SYSTEM
DISCUSSION
 Brassica rapa is a weedy, selfincompatible winter annual
• Heritability in direction of early flowering is comparable to that in direction of late flowering (Fig
3a). This is surprising given the wide variation in offspring FT of late flowering mothers (Fig 2a).
 Native range is Eurasia, but it is
found widely across Canada as an
naturalized weed
• Re-examining Fig 2a, increasing offspring variance does not necessarily indicate a reduced capacity
to respond to selection. Heritability is constant and the mean FT is still higher.
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 Our populations were founded
from seeds collected at a Quebec
site in 2009
• Both mean and variance of flowering time are greater in the late selection lines than in the early
selection lines (Fig 3b), consistent with the pattern observed in Fig 2b.
• The slight asymmetry in HA lines in Fig 3a may be due to underlying genetic asymmetry, if
hyperassortative mating exposes rare recessive alleles for late flowering.
 Populations subjected to a
specific mating scheme for 2
generations prior to experiment
•A strong positive relationship exists between flowering day and plant size. This suggests that plant
life-history traits may be constrained by genetic and environmental correlations resulting in an
overall positive phenotypic correlation.4
METHODS
ACKNOWLEDGEMENTS
•Hyperassortative (HA) plants were pollinated
with individuals flowering on the same day.
Naturally assortative (NA) plants were
pollinated within groups.
•At maturity, fruit was harvested and plant size
(stem diameter and # of leaf scars) was
recorded.
We would like to thank all the members of the Weis lab for their assistance and
support throughout this project. Special thanks go to Jennifer Ison, Geoffrey
Legault and Susana Wadgymar for their input in all facets of the experiment.
We also wish to extend our thanks to Alex Levit and Karen Bai for their help in
the greenhouse. We are also extremely grateful to the many undergraduate
volunteers who assisted with data collection and entry. Lastly, we wish to thank
both Andrew Petrie and Bruce Hall for their assistance in caring for the
greenhouse plants.
Xind. = FTind – FTctrl
Sline = Xselected – Xpopulation
Rline = Xline – Xctrl
GEN 1
BASE
Early
15%
CTRL
Late
15%
GEN 2
REFERENCES
1.
2.
3.
4.
5.
Montague, J. L., S. C. H. Barrett, and C. G. Eckert. 2008. Journal of Evo. Bio. 21:234-245.
Franks, S. J., S. Sim, and A. E. Weis. 2007. Proc. of the Nat. Acad. of Sci. USA 104:1278-1282.
Burgess, K. S., J. R. Etterson, and L. F. Galloway. 2007. Heredity 99:641-648.
Dorn, L.A., and T. Mitchell-Olds. 1991. Evolution 45: 371-379.
printed by
Frankham, R. 1990. Genet. Rsrch 56:35-42.
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