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REPRODUCTIVE BIOLOGY OF
BITTERBRUSH: INTERACCESSIONAL
HYBRIDIZATION OF PLANTS
GROWN IN A COMMON GARDEN
R. L. Pendleton
E. D. McArthur
In May 1989, we applied the following treatments to
separate branches on six individuals of each of the four
populations: control (open pollination), bagged (self pollination), bagged and emasculated, bagged with self pollen
added, and bagged with nylon net (wind pollination). In
May 1990, treatments consisted of reciprocal hybridization among the four core populations (two separate donor
bushes for each recipient bush), one self pollination, one
open pollination, one nearby random antelope bitterbrush
pollen donor, one 'Lassen' (Shaw and Monsen 1986) antelope bitterbrush pollen donor, and two Stansbury cliffrose
(from St. George and Provo Canyon, UT) pollen donors for
a total of 12 treated branches per bush for each of six
bushes per accession.
Except for open- and wind-pollination treatments, mass
pollination was accomplished at anthesis in white paper
sacks, which were used to isolate the treated branches.
Cloth net bags replaced the paper sacks and were added
to the open pollination treatments to protect the developing fruits from insect predation and to collect ripening
fruits. After harvest, receptacles and fruits were counted
and sorted into three developmental classes. Fruit and
seed size measurements were also recorded. Data were
analyzed using the General Linear Models procedure on
Bitterbrush (Purshia spp.) and cliffrose (Cowania spp.)
comprise a widespread western North American species
complex, a complex capable of interspecific gene exchange.
Various species of this group are important in landscape
dominance, wildlife habitat, browse for wild and domestic
ungulates, contribution to the rare species flora, and wildland nitrogen fiXation. Antelope bitterbrush (P. tridentata) and Stansbury clift'rose (C. stansburiana) are the
most common and widespread members of this species
complex.
This study looks at the reproductive system of antelope
bitterbrush, testing whether individual plants of this species are self-incompatible and whether pollination is facilitated by insect vectors. We also examine intra- and
interpopulation compatibility of four core antelope bitterbrush accessions in a diallelic crossing design supplemented by additional antelope bitterbrush and Stansbury
clift'rose pollen donor accessions. One of the four core
accessions is of hybrid origin (P. tridentata xC.
stansburiana).
Specific questions addressed by this study are:
1. Are individual plants of Purshia self-incompatible?
What role does wind play in the pollination of this species?
2. What fruiting characteristics are attributable to maternal or paternal effects? Are there differences among
populations?
3. Are crosses using within-accession pollen more successful than· crosses between accessions? What are the
characteristics of interspecific crosses?
SAS.
POLLINATION
Bitterbrush is highly self-incompatible. In year 1 of the
study, 1.06 percent of the flowers on branches that were
bagged to exclude pollinators set fruit. Similar results
were recorded in year 2, with self pollination producing a
1.59 percent fruit set. In contrast, an average 82.9 percent of open-pollinated flowers produced fruit.
Wind pollination within the high-density common garden was nearly as effective as open pollination (77 percent
fruit set). Fruit set in the hybridization treatments averaged less than 70 percent. The reduction in fruit set with
treatment is likely due to a "bag effect" (not all flowers
were open or accessible to pollen introduced to the bag).
METHODS
Four core antelope bitterbrush populations (Bryce
Canyon, UT; Canyon Mountains, UT; Fairview, UT;
Thoreau, NM) are among some 60 antelope bitterbrush
and Stansbury cliffrose accessions growing in a Springville,
UT, uniform garden. The Thoreau accession is of hybrid
origin.
MATERNAL AND PATERNAL
EFFECTS ON FRUITING
CHARACTERISTICS
Poster paper presented at the Symposium on Ecology, Management,
and Restoration of Intennountain Annual Rangelands, Boise, ID,
May 18-22, 1992.
R. L. Pendleton is Ecologist and E. D. McArthur is Supervisory
Research Geneticist at the Shrub Sciences Laboratory, Intennountain
Research Station, Forest Service, U.S. Department of Agriculture,
Provo, UT.
Differences between the four maternal populations were
tested using individual plants as the error term of the
model. Significant differences were found in seed weight,
268
Table1-Attalned significance values from ANOVAs for maternal and paternal effects. NS =nonsignificant
Mean seed
weight
Mean fruit
length
Percent fruit
Initiated
Percent fruit
developed
Percent fruit
matured
Percent fruit
aborted
0.0007
0.0009
0.0039
0.0047
0.0113
0.0018
Maternal
parent
.0001
.0001
.0001
.0001
.0001
NS
Paternal
population
NS
.0135
NS
.0433
.0559
NS
Paternal
parent
NS
.0008
NS
NS
NS
NS
Maternal
population
CONCLUSIONS
fruit length, percent fruit initiated, percent fruit developed, percent fruit matured, and percent fruit aborted
(table 1). The Thoreau population, which contains
introgressed Cowania genes, was significantly different
from other populations in all variables. The smaller seeds,
multiple seeds per receptacle, and longer tails on the
fruits, traits it shares in common with Cowania, attest to
its hybrid origin. The Bryce Canyon population had significantly less fruit abortion than either West Fairview or
Canyon Mountain populations.
Maternal parent plant had a profound effect on fruiting
characteristics, being highly significant for all variables
except fruit abortion. For nonintrogressed populations
(Thoreau omitted), seed weight is almost exclusively
determined by the maternal parent. Maternal-parent
effects include both genetic and environmental
components.
Three variables show some effect attributable to
paternal-parent population. The strongest effect is observed in fruit length. Fruits developing from cliffrose
(Cowania) pollen tended to be longer than other sources.
Cliffrose crosses also tended to produce a lower percent
seed set than other pollen treatments. West Fairview produced the highest percent fruit set.
Individual pollen parent effects were highly significant
for the fruit length variable. Again, cliffrose pollen sired
the longest fruits. The two Canyon Mountain pollen
sources were significantly different from each other, one
plant siring fruits nearly as long as those produced by
cliffrose pollen, the other siring short fruits.
1. Bitterbrush is self-incompatible but widely fertile on
an intra- and interpopulational basis, including hybridization with cliffrose. Cross-fertilization and hybridization
for breeding purposes can be accomplished with relative
ease. Where plants are closely spaced, wind may play a
significant role in effecting pollination. Pollen limitation
should not pose a significant problem to fruit production
in an orchard setting.
2. The maternal parent has the greatest effect on fruiting characteristics, being highly significant for nearly all
variables examined in this study. The maternal parent
largely determines seed weight, which in tum affects
seedling survival and performance. Mean seed weight
in a given lot can likely be increased through selective
propagation of large-seeded mother plants in a garden
setting or, alternatively, by selective field harvesting.
3. Paternal effects, though less substantive, are also
important in fruit development. This study demonstrates
morphologic effects attributable to pollen source. Studies
are now planned that will examine possible paternal and
maternal effects on seed germination and seedling establishment. The demonstration of maternal (at least some
of which is due to genetics) and paternal (all of which is
due to genetics) effects on seed development and morphology confirms the presence of a large genetic pool amenable to breeding programs.
4. Intra- and interpopulational crosses mature the
same proportion of seed, albeit through different means.
Within-population crosses initiate more fruit, but also
abort a higher proportion. This further supports the
conclusion that fruit production in bitterbrush is largely
a function of resource availability, rather than pollen
limitation.
INTRA· VS. INTERPOPULATION
CROSSES
=
Although significant only at the p 0.1level, intra- and
interpopulational comparisons demonstrate an interesting
trend that may be worthy of more detailed investigation.
Within-population crosses initiate (82.5 percent vs. 74.0
percent) and develop (60.0 percent vs. 50.6 percent) a higher
percentage of fruit, but also abort (16.5 percent vs. 12.0 percent) a higher percentage of developing fruit before reaching
final maturity. Intra- and interpopulational crosses did
not differ in the percent of mature fruit produced.
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