Original ResearchTrait Response and Change in Genetic Variation upon Selection for Spike Number in Salina Wildrye
Introduction
Salina wildrye (Leymus salinus [M.E. Jones] Á. Löve = Elymus salinus M. E. Jones) is a moderately rhizomatous, cool-season perennial grass native to the Intermountain West (Monsen et al., 2004, p. 381 − 382). This grass could be used for restoration of harsh, arid, disturbed sites such as those that have been developed for oil or gas production. It may grow on a variety of soil textures ranging from rocky to loams and clays, occupying hillsides, alluvial fans, plateaus, bluffs, canyons, and montane areas as part of salt desert shrubland, sagebrush-grass, mountain mahogany, aspen, and conifer plant communities (Baker and Kennedy, 1985, Vallentine, 1989, Monsen et al., 2004, p. 381 − 382). Leymus salinus ssp. salinus is prevalent east of the Wasatch Mountains in eastern Utah, western Colorado, and southwestern Wyoming (Barkworth and Atkins, 1984, Fig. 6) and may be found in association with the saltbush shrub species Atriplex confertifolia and A. gardneri (Baker and Kennedy, 1985). A second subspecies of L. salinus, ssp. salmonis, occurs west of the Wasatch Mountains in eastern Idaho, western Utah, and Nevada. While ssp. salinus is relatively frequent within its range, the distribution of ssp. salmonis is much spottier (Atkins et al., 1984). As far as is known, ssp. salmonis is tetraploid (2n = 28), while ssp. salinus may be tetraploid, hexaploid (2n = 42), or octoploid (2n = 56) (Atkins et al., 1984). A third subspecies, ssp. mojavensis, occurs in southern California and Arizona (Barkworth and Atkins, 1984). Leymus salinus and L. cinereus have been shown to be the parents of L. ambiguus (Culumber et al., 2011, Fig. 2), which occurs on the eastern slopes of the Rocky Mountains in either tetraploid or octoploid states (Atkins et al., 1984).
Because no plant materials of salina wildrye have been released for commercial seed production, only wildland-collected seed has been available for seeding operations, which means that seed quality is often poor, seed prices are high, and seed availability is limited. However, 9043501, an octoploid population collected in northeastern New Mexico (Colfax County), has been identified by the Upper Colorado Environmental Plant Center in Meeker, Colorado as a promising plant material (Monsen et al., 2004). This population keys to L. salinus ssp. salinus because it possesses neither the pubescent basal leaves of ssp. salmonis nor the flat leaf blades of ssp. mojavensis (Barkworth and Atkins, 1984); that is, its leaves are glabrous and strongly involute.
The greatest limiting factor to the adoption of this species in the seed trade is its poor seedling establishment and low seed yield relative to other cool-season perennial rangeland grasses. Low seed yield is primarily due to the production of relatively few inflorescences (spikes) per plant. For this reason, we conducted two cycles of phenotypic recurrent selection for increased spike number in an attempt to increase seed production potential of 9043501. As L. salinus is an allogamous species, hybridization occurs between plants each generation, forcing genetic recombination and making it a logical candidate for recurrent selection. Before this selection, we had conducted two cycles of selection for salinity tolerance, but response to that earlier selection is not germane to this study. In an evaluation of response to selection at Millville, Utah, we included the octoploid 9043501, the four cycles (populations) of selection on 9043501, and Lakeside C3, a tetraploid ssp. salmonis population used for comparison with 9043501.
We had three objectives in this study. Objective 1 was to compare the L. salinus ssp. salmonis Lakeside C3 population (2n = 4x = 28) to the L. salinus ssp. salinus 9043501 C0 population (2n = 8x = 56) (henceforth 9043501) for the previously mentioned traits. Objective 2a was to evaluate the effect of two cycles of selection on 9043501 for spike number per plant (C3, C4) on spike number per plant, seed yield per plant, and the two other components of seed yield, seeds per spike and individual seed mass. Objective 2b was to evaluate the effects of selection on 9043501 on two vegetative traits, dry matter per plant and canopy height, and two germination traits, percentage and rate. Objective 3 was to measure genetic similarity within 9043501 and each of four cycles of selection in 9043501, two (C1, C2) for salinity tolerance and two (C3, C4) for spike number. This was done to determine whether genetic variation had been lost from the 9043501 base population as a consequence of artificial selection. Genetic variation can be lost from populations subjected to selection primarily due to genetic drift, a process by which a population is genetically narrowed due to a relatively small number of individuals, leading to unavoidable mating of relatives and consequential inbreeding.
Section snippets
Selection History
Four cycles of selection have been practiced on 9043501. Selection was applied for tolerance to salinity in the first two cycles and for spike number (ocular estimate) in the latter two cycles. For C1, seeds were germinated in an EC = 24 solution in the laboratory (Peel et al., 2004). Seeds that germinated were planted individually into 1 020 silica sand-filled cones in a greenhouse and subjected to a salinity protocol (dunked in saline solution 2 × per week), escalating from EC = 12 (17
Comparison of Lakeside and 9043501
Performance of Lakeside was similar to that of 9043501 early in the trial, but as time progressed differences became more conspicuous, with the notable exceptions of spike number per plant and seed mass (Table 1). Spike number in 2013 was 49.7% lower (P < 0.05) for C0 than Lakeside, while in 2014 and 2015 spike numbers of 9043501 and Lakeside were similar (P > 0.10). Nevertheless, Lakeside’s higher 2013 spike number did not translate into a seed yield advantage (P > 0.10) in that year. In 2014,
Lakeside versus 9043501 Comparison
Objective 1 was to compare Lakeside with 9043501. In 2013, the first seed-production yr, the two populations were similar (P > 0.10) for seed yield (see Table 1). This was because, despite double the number of spikes for Lakeside relative to 9043501 (P < 0.0001), the latter’s greater number of seeds per spike (P < 0.05) compensated for its lower spike number (see Table 1). Beginning in 2014, however, relative seed yield of the two populations began to diverge (P < 0.10) in favor of 9043501,
Implications
Priming natural selection in the wild with adaptive variation from an ex situ source, such as the plant material developed here, is a strategy of “assisted evolution” (Jones and Monaco, 2009). Assisted evolution considers both empirical performance and genetic identity. Here we used the age-old plant breeding technique of artificial selection to create material that features the “genetic background” of the 9043501 C0 base population yet displays greater fecundity. Restoration practitioners
Acknowledgments
Thanks to Kevin Jensen and the Upper Colorado Environmental Plant Center for providing seed of the Lakeside C3 check population and 9043501, respectively. Thanks also to two anonymous reviewers for their insightful comments.
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