Pollen Morphology and Fertilization in Leptosiphon Species
Aaron Wallace
Biology Department
East Carolina University
Greenville, North Carolina 27858
[email protected]
by Aaron Wallace
Spring 2009 Scanning Electron Microscopy Class Project
ABSTRACT
DISCUSSION
Leptosiphon parviflorus and Leptosiphon jepsonii are
two species of annual dicots that are native to
California. They exhibit different reproductive
strategies. L. parviflorus is a self-incompatible species
(cannot self-fertilize) that is fertilized through wind or
insect pollination (3). L. jepsonii is a species that is
initially self-incompatible, but will self-fertilize if
cross-pollination does not occur (2). Pollen grains
were examined by environmental scanning electron
microscopy (ESEM) to determine if structural
differences in the pollen of the two species could
affect the plant’s ability to self-fertilize. Both species
of pollen displayed similar morphology and
comparable size, which suggests that pollen grain
morphology is not the limiting factor for selffertilization.
METHODS
Leptosiphon
flowers
from the jepsonii and
paraflorus species were collected during blooming
cycles. The flowers were collected immediately before
examination and prepared for ESEM analysis. Petals
were dissected from L. paraflorus to reveal the pistils
and stamens, which were generally obscured from view
by the upturned petals. L. jepsonii pistils and stamens
were already exposed for examination due to the
downturn of its petals.
Specimens were loaded directly on the tab and
examined under a pressure of 5 Torr at 2º C, which
produced 95% relative humidity. Images were taken of
multiple flowers from several species with particular
emphasis on images of pollen and its interaction with
the pistils of the flower.
Leptosiphon flowers were viewed under 25kV and a
spot size of 4. A standard working distance of 10-12 mm
was used for optimal image quality
Pollen grains were typically free from charging and
artifacts. The stamen and pistil, as well as the petals,
were subject to charging and artifacts from electron
beam exposure. Pressure, spot size, beam kV, and scan
rate were adjusted as needed to increase sample stability
Less stable structures were examined at lower
resolutions and higher scan rates in contrast to pollen,
which was more resistant to
the electron beam.
SUMMARY of RESULTS
Leptosiphon jepsonii is generally cross-pollinated (2). Its
downturned petals (Fig. 2) facilitate pollination by wind or insects.
However, if cross-pollination does not occur, L. jepsonii will selfpollinate (1). The mechanism that promotes this switch in
reproductive methodology is not currently understood (pers. comm.
C. Goodwillie).
Leptosiphon parviflorus is a self-pollinating species. It has
upturned petals (Fig. 3).
Pollen morphology was thought to possibly have an effect on
self- versus cross-pollination. However, visual inspection of pollen
2
grains indicated no significant structural differences.
Specimens with non-spherical shapes and depressions (Fig. 4, 5)
may result from age or environmental factors. Depressions were not
found on grains that had pollen tube growth (Figs. 8-9). The pollen
grains with pollen tube growth had consistent morphological features.
The grains were void of spikes or structures to account for differences
in the preferred method of fertilization between L. jepsonii and L.
parviflorus.
Pollen grains found within each flower showed variations in size,
smoothness, and overall shape. Small, smooth pollen grains were
found in both species (Fig. 5, 6, 7). These variants were not isolated to
3
a single location within either species of flower.
1
between species.
• Older pollen grains showed more depressions
and irregularities (fig. 4).
• Structural differences do not explain differences
in pollination strategy.
• Differences in reproductive strategy may be due
to plant response in stamen or pistil.
• Age of flower may influence variable
reproductive strategies (2).
Pollen Tube
8
9
Stigma
Pollen grains from L. jepsonii and L. parviflorus with
visible pollen tube growth.
Fig. 8: L. jepsonii pollen grain with diameter of 51 µm.
Fig. 9: L. parviflorus pollen grain with diameter of 41
µm .
Pollen
REFERENCES
5
4
Fig. 4: L. jepsonii pollen grains on stamen .
Fig. 6: L. jepsonii pollen with pollen tubes .
Fig. 5: L. parviflorus pollen grains.
Fig. 7: L. jepsonii stamen and filament with pollen on top of petal.
1. Goodwillie, C. 2000. Inbreeding Depression and Mating Systems in Two Species of
Linanthus. Heredity, 84: 283-293.
2. Goodwillie, C. 2004. Transient Self-Incompatibility Confers Delayed Selfing in
Leptosiphon jepsonii. International Journal of Plant Science, 165: 387-394.
3. Goodwillie, C. 1999. Wind Pollination and Reproductive Assurance in Linanthus
parviflorus, a Self-Incompatible Annual. American Journal of Botany, 86: 948-954.
ACKNOWLEDGEMENTS
Stigma
Pollen Tube
Fig. 1. Scanning Electron Microscope
stage with Leptosiphon specimen.
• Pollen grains had similar shapes and structures
6
7
I would like to acknowledge the East Carolina University Biology
Department for providing its facilities for research and Dr. Carol
Goodwillie and the Botany lab at ECU for providing Leptosiphon
species for examination. Also, we would like to acknowledge Dr.
Thomas Fink in the Department of Biology Imaging Facility for his
assistance and instruction with the scanning electron microscope and
ESEM techniques. Special thanks to both professors for providing
indispensible support and guidance during this project.