Comparative Mapping of Growth Habit, Plant Height, and Flowering QTLs in Two Interspecific Families of Leymus Steven R. Larson,* Xiaolei Wu, Thomas A. Jones, Kevin B. Jensen, N. Jerry Chatterton, Blair L. Waldron, Joseph G. Robins, B. Shaun Bushman, and Antonio J. Palazzo ABSTRACT Leymus cinereus (Scribn. & Merr.) A ´ . Lo ¨ ve and L. triticoides (Buckley) Pilg. are tall caespitose and short rhizomatous perennial Triticeae grasses, respectively. Circumference of rhizome spreading, proportion of bolting culms, anthesis date, and plant height were eval- uated in two mapping families derived from two interspecific hybrids of L. cinereus Acc:636 and L. triticoides Acc:641 accessions, backcrossed to one L. triticoides tester. Two circumference, two bolting, and two height QTLs were homologous between families. Two circumference, seven bolting, all five anthesis date, and five height QTLs were family specific. Thus, substantial QTL variation was apparent within and be- tween natural source populations of these species. Two of the four circumference QTLs were detected in homoeologous regions of linkage groups 3a and 3b in both families, indicating that one gene may control much of the dramatic difference in growth habit between these species. A major height QTL detected in both families may correspond with dwarfing mutations on barley 2H and wheat 2A. The L. cinereus parent contributed negative alleles for all four circumference QTLs, five of nine bolting QTLs, two of five anthesis date QTLs, and one of seven height QTLs. Coupling of synergistic QTL allele effects within parental species was consistent with the divergent growth habit and plant height of L. cinereus and L. triticoides. Conversely, antagonistic QTL alleles evidently caused transgressive segregation in reproductive bolting and flowering time. L EYMUS wildryes are long-lived Triticeae grasses, closely related to wheat (Triticum spp.) and barley (Hordeum vulgare L.). The genus Leymus includes about 30 species distributed throughout temperate re- gions of Europe, Asia, and the Americas (Dewey, 1984). More than half of all Leymus species are allotetraploids (2n 5 4x 5 28), but octoploid (2n 5 8x 5 56) and do- decaploid (2n 5 12x 5 84) variants may arise from inter- specific hybrids (Anamthawat-Jo ´ nsson and Bo ¨ dvarsdo ´ ttir, 2001) or autoduplication within species. These cool- season grasses display remarkable variation in growth habit and stature with unusual adaptation to harsh polar, desert, saline, and erosion-prone environments. Leymus triticoides (creeping or beardless wildrye) and L. cinereus (basin wildrye) are closely related but mor- phologically divergent North American range grasses. Aggressive rhizomes and adaptation to poorly drained alkaline sites, primarily within the western USA, charac- terize sod-forming L. triticoides (0.3–0.7 m). Conversely, L. cinereus is a tall (up to 2 m) conspicuous bunchgrass adapted to deep well-drained soils from Saskatchewan to British Columbia, south to California, northern Arizona, and New Mexico, and east to South Dakota and Minne- sota. Most populations of L. cinereus and L. triticoides are allotetraploids; however, octoploid forms of L. cinereus are typical in the Pacific Northwest. Basin wildrye, and octoploid giant wildrye [L. condensatus (J. Presl) A ´ . Lo ¨ ve], are the largest cool-season bunch grasses native to western North America. Artificial hybrids of L. cinereus , L. triticoides , and other North American Leymus wildryes display regular meiosis and stainable pollen (Stebbins and Walters, 1949; Dewey, 1972; Hole et al., 1999). Both L. cinereus and L. triticoides are highly self-sterile (Jensen et al., 1990) and hybridize with each other in nature. These species are naturally important forage and hay grasses in the Great Basin and other regions of western North America. Growth habit is a highly variable and ecologically important trait in perennial grasses. Aggressive rhi- zomes characterize quackgrass [Elymus repens (L.) Desv. ex Nevski] and johnsongrass [Sorghum bicolor (L.) Moench], which rank among the world’s worst perennial grass weeds (Holm et al., 1977). In general, caespitose (i.e., growing in bunches or tufts) and rhizomatous grasses dominate semiarid and mesic grasslands, respectively (Sims et al., 1978). Nutrient islands beneath caespitose grasses may also contribute to clone fitness in this growth form in both mesic and semiarid communities, whereas the distribution of rhizomatous grasses may be restricted to microsites characterized by higher soil organic carbon and nitrogen concentrations (Derner and Briske, 2001). We have observed L. cinereus and L. triticoides growing in close proximity in mixed stands, at several disturbed sites, with no apparent difference in microhabitat. Conversely, we have observed L. triticoides in riparian or wet zones and L. cinereus inhabiting dry adjacent uplands, restricted to seemingly different natural microhabitats. In any case, L. cinereus and L. triticoides display profound differences in growth habit. Lateral branches of L. cinereus grow strictly upward, often within the lower leaf sheaths as tillers, whereas the lateral branches of L. triticoides fre- quently grow horizontally or downward under the soil surface as rhizomes. In addition to obvious morphological S.R. Larson, T.A. Jones, K.B. Jensen, N.J. Chatterton, B.L. Waldron, J.G. Robins, and B.S. Bushman, USDA-ARS, Forage and Range Research Lab., Utah State Univ., Logan, UT 84322-6300; X. Wu, Univ. of Missouri-Columbia, Columbia, MO 65211; A.J. Palazzo, U.S. Army Corps of Engineers, Engineering Research and Development Center-Cold Regions Research and Engineering Lab., Hanover, NH 03755-1290. Received 13 Dec. 2005. *Corresponding author (stlarson@ cc.usu.edu). Published in Crop Sci. 46:2526–2539 (2006). Genomics, Molecular Genetics & Biotechnology doi:10.2135/cropsci2005.12.0472 ª Crop Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA Abbreviations: AFLP, amplified fragment length polymorphism; ANTH, anthesis date; BOLT, reproductive bolting; CIRC, plant circum- ference; HGHT, plant height; IM, interval mapping; MQM, multiple- QTL model; QTL, quantitative trail locus (i); RFLP, restriction fragment length polymorphism; rMQM, restricted MQM mapping; STS, sequence- tagged site; SSR, simple-sequence repeat. Reproduced from Crop Science. Published by Crop Science Society of America. All copyrights reserved. 2526 Published online November 21, 2006