Identification of Male-Specific AFLP Markers in Dioecious Texas Bluegrass

is more common among the dicots than the monocots (Renner and Ricklefs, 1995). Dioecy is a breeding system that promotes cross-pollination in Poa is a large, diverse genus that belongs to the subplants. The transfer of this trait into economically important selfpollinated cereal crops would revolutionize the production of hybrids family Pooideae of the Poaceae. Several species are in these species and provide a means for increasing yields because of complex polyploids with a range of breeding systems, heterosis. Texas bluegrass (Poa arachnifera Torr.) (2n 8x 56) including apomixis and dioecy, that promote interspeis a polymorphic dioecious species that provides an opportunity to gecific hybridization and introgression (Muntzing, 1940; netically map the dioecy locus. In this study, amplified fragment length Clausen, 1961). Because of this, it is frequently difficult polymorphism (AFLP)-based linkage maps were constructed for both to taxonomically distinguish one species from another. the maternal and paternal plants used to develop a mapping populaTexas bluegrass (2n 8x 56) is a dioecious, perennial tion of Texas bluegrass. The maternal map contained 126 single dose cool-season grass that is native to southern Kansas, restriction fragments (SDRFs), 31 linkage groups, 1744 cM, and an Oklahoma, western Arkansas, and most of Texas average marker spacing of 13.8 cM. The paternal map contained 210 (Hitchcock, 1950; Gould, 1975). It is drought and heat SDRFs, 46 linkage groups, 2699 cM, and an average marker spacing of 12.9 cM. Approximately 76 to 81% of the Texas bluegrass genome tolerant and produces high quality forage in regions was covered. Two AFLP markers (txbg7 and txbg154) mapped equiwhere other cool-season temperate grasses, such as tall distantly (9.5 cM) on opposite sides of the dioecy locus (PDio1) on fescue (Festuca arundinacea Schreb.), are not sustainthe paternal map. These markers provide a preliminary tool for studyable (Read et al., 1997). The grass grows throughout ing sex determination and a framework for further characterization the winter but is usually dormant during the summer of the genomic region conferring dioecy in Texas bluegrass. (Magness et al., 1971). However, it produces only limited quantities of seed that are covered with woolly hairs that are difficult to remove. Consequently, establishA 90% of the flowering plants in the ment of stands for agricultural use is difficult. The speworld are hermaphroditic in that the female and cies has been successfully crossed with Kentucky bluemale reproductive organs are in the same flower (Ainsgrass (Poa pratensis L.) in an effort to combine heat worth, 2000; Charlesworth, 2002). However, in some and drought tolerance of Texas bluegrass with high forspecies, hermaphroditic flowers have evolved several age and turf quality of Kentucky bluegrass (Vinall and mechanisms that increase cross-pollination, which avoids Hein, 1937; Read et al., 1999; Bonos et al., 2000; Abrainbreeding depression and promotes heterozygosity, geham et al., 2004). ‘Reveille’, the first Texas bluegrass netic variability, and genetic exchange (Dellaporta and Kentucky bluegrass hybrid cultivar released, is a heat Calderon-Urrea, 1993). These mechanisms include protolerant turfgrass adapted to the southwestern USA togyny, chasmogamy, heterostyly, self-incompatibility, (Read et al., 1999). Hybrids between dioecious Texas self-sterility, entomophily, anemophily, and hydrophily. bluegrass and apomictic Kentucky bluegrass are facultaThe remaining flowering plants have evolved physically tive apomicts and demonstrate the reproductive versaseparated, unisexual flowers. Approximately half of these tility within Poa. are monoecious, and the male and female reproductive Although little molecular information is available reorgans develop in separate flowers on the same plant. garding Texas bluegrass, a study using AFLP markers Monoecy prevents intraflower self-pollination but does investigated the diversity within the species and deternot necessarily prevent intraindividual self-pollination mined that genetic mapping is feasible (Renganayaki et (Charlesworth, 2002). The remaining plants are dioecious al., 2001). Dioecy is a trait that is of considerable interest in that individual plants within the same species have in mapping, and molecular markers have been linked either male or female flowers. Dioecy prevents intrainto dioecy in a number of angiosperms (Ainsworth, 2000; dividual self-pollination and is one of the most extreme Charlesworth, 2002; Vyskot and Hobza, 2004). Howinbreeding avoidance mechanisms (Ainsworth, 2000). ever, most of these are dicots and little has been done Dioecy occurs throughout the angiosperms; however, it in the monocots, especially members of the Poaceae family. According to Watson and Dallwitz (1992 onK. Renganayaki, R.W. Jessup, and M.A. Hussey, Dep. Soil & Crop ward), there are 22 grass genera that have at least one Sciences, Texas A&M Univ., College Station, TX 77843-2474; B.L. dioecious species, and of these, a molecular study adBurson, USDA-ARS, Crop Germplasm Research Unit, 430 Heep dressing sex determination has been conducted for only Center, Texas A&M Univ., College Station, TX 77843-2474; J.C. one species. Eppley et al. (1998) reported a single ranRead, Texas A&M Univ. Research and Extension Center, 17360 Coit Road, Dallas, TX 75252-6599. Received 14 Feb. 2005. *Corresponding dom amplified polymorphic DNA (RAPD) marker author ([email protected]). linked to the dioecy trait in salt grass [Distichlis spicata Published in Crop Sci. 45:2529–2539 (2005). Crop Breeding, Genetics & Cytology Abbreviations: AFLP, amplified fragment length polymorphism; cM, centimorgan; PCR, polymerase chain reaction; RAPD, random amplidoi:10.2135/cropsci2005.0144 © Crop Science Society of America fied polymorphic DNA; SDRF, single dose restriction fragment; TBF, Texas blue forage; TIFF, tagged image file format. 677 S. Segoe Rd., Madison, WI 53711 USA 2529 Published online October 27, 2005