1. Sources
of
varia-on
in
pollen
deposi-on
and
pollen
tube
a4ri-on
in
two
Clarkia
species:
the
effects
of
style
length
and
seasonal
-ming
Arrash
Moghaddasi,
Brandon
Wallace,
Alexandra
Bello,
Alisa
Hove,
and
Susan
J.
Mazer
Department
of
Ecology,
Evolu-on
and
Marine
Biology,
UCSB
Introduction
Due to their sessile nature, plants differ from animals in that plants cannot actively choose
their mates. Nevertheless, after pollination, male gametophytes may compete with each
other to reach the base of the style, and male-female interactions may influence which
pollen tubes are most likely to succeed in reaching the ovary and in fertilizing the ovules.
Pollen tube attrition — or rate of failure of pollen grains or pollen tubes to reach the ovary —
may be the result of interference among competing pollen grains (and pollen tubes) or the
result of poor environmental conditions within the maternal flowers. In either case, following
pollen germination, it is unlikely that all pollen grains that land on a stigma will be successful
in fertilizing an ovule. In this study, we examined several intrinsic and extrinsic factors that
may influence pollen attrition rates in natural populations of the annual wildflower species,
Clarkia unguiculata and C. xantiana subspecies xantiana. Sources of variation were
explored by asking the following questions:
1. Does style length have a significant effect on the amount of pollen received?
2. Does time of floral sampling affect the amount of pollen received or the rate of pollen
tube attrition?
3. Do species differ in the patterns observed?
Results
The effects of style length on pollen load
The relationship between pollen deposition and style length differed between
species and sampling dates. Where style length predicted the number of pollen
grains deposited on stigmas, however, the relationship was always positive;
flowers with relatively long styles received more pollen than those with
shorter styles.
In C. unguiculata, style length did not have a statistically significant effect on
pollen load among styles sampled in 2009 (Fig. 1A) (n = 70, r 2 = 0.0103, P =
0.403) or in early 2010 (Fig. 1B) (n = 213, r 2 = 0.0119, P = 0.112). Style length
did have a significant and positive effect, however, in late 2010 (Fig. 1C) (n =
213, r 2 = 0.344, P < 0.001*). In C. xantiana ssp. xantiana, style length had a
significant and positive effect on pollen load in 2009 (Fig. 1D) (n = 239, r 2 =
0.0515, P < 0.004*). In 2010, style length did not have a significant effect on
pollen load early in the season (Fig. 1E) (n = 90, r 2 = 0.0211, P = 0.172), but did
have a significant positive effect late in the season (Fig. 1F) (n = 172, r 2 =
0.0715, P < 0.004*).
Conclusion
In both taxa style length was found to have a significant positive effect on pollen load late in the season in 2010; in 2009, the relationship was positive in C. xantiana
ssp. xantiana. This correlation may be due to pollinator preference for (and greater visitation of) larger flowers or by more frequent contact between floral visitors
and the stigmas of longer-style flowers.
Timing of style harvest was found to influence attrition rate in C. unguiculata, in which flowers harvested later in the season had higher rates of attrition from the
stigma to the stigma-style junction and from the stigma to the style base. No effect on pollen tube attrition rates was found in response to the timing of style harvest
in C. xantiana ssp. xantiana. To our knowledge, this is the first study to demonstrate a consistent effect of style length on pollen deposition and to detect
seasonal changes in pollen attrition rate.
Temporal differences in pollen tube attrition
Pollen tube attrition from the stigma to the SSJ: The rate of pollen tube attrition
during germination and early pollen tube growth differed between styles sampled
Early vs. Late in C. unguiculata but not in C. xantiana. In C. unguiculata, the
attrition rate increased significantly between samples collected Early and Late in
the season (early mean = 0.579, late mean = 0.663; n = 299, r 2 = 0.0243, P =
0.0069*) (Fig 4A). There was no significant effect of harvest date on the attrition
rate in C. xantiana ssp. xantiana (early mean = 0.584, late –mean = 0.554; n =
261, r 2 = 0.00309, P = 0.371) (Fig. 4B).
Pollen tube attrition from the SSJ to the style base: The timing of style harvest
had no significant effect on the mean attrition rate from the SSJ to the style base
on either taxon (Fig. 5). There was no significant effect seen in C. unguiculata
(early – mean = 0.407, late – mean = 0.372; n = 300, r 2 = 0.00374, P = 0.291)
(Fig. 5A) or C. xantiana ssp. xantiana (early – mean = 0.335, late – mean =
0.318; n = 260, r 2 = 0.00108, P = 0.598) (Fig. 5B).
Pollen tube attrition from the stigma to the style base: The timing of style harvest
had a significant effect on pollen tube attrition over the entire length of the style in
C. unguiculata (early – mean = 0.771, late – mean = 0.811; n = 299, r 2 = 0.0198,
P = 0.0149*) (Fig. 6A) but not in C. xantiana ssp. xantiana (early – mean = 0.721,
late – mean = 0.673 ; n = 261, r 2 = 0.00422, P = 0.2956) (Fig. 6B).
Clarkia unguiculata Clarkia xantiana spp. xantiana
Images courtesy of Google
Dyed callose plugs at SSJ of C.x.x. sample Dyed callose plugs at style base of C.x.x. sample
Considering that this was my Senior thesis project, my role
was to oversee that all pollen deposition and pollen tube
counts were performed.
Describe your experience as a research assistant
- I learned techniques that I would have never learned
outside of the Mazer lab including: fluorescence
microscopy, style dissection, and pollen tube staining and
visualization.
- I am deeply indebted to Professor Susan J. Mazer and
my colleagues for committing their time to revise my
thesis.
- I can apply organizational and time management skills for
future research endeavors. Since this was my first time
performing a research project on my own, I was allowed to
set my own time schedule. I think this helped me make
good use of my time in the lab. Writing a thesis taught me
how to write all the components that go into a real article.