ods. In Tennessee, genetic gain in yield improvement was 7.2 kg ha Ϫ1 yr Ϫ1 as calculated by Culp and GreenThe New Mexico cotton breeding program was established in 1926 (1992) on the basis of the data from Hoskinson and Stewand has been led by five generations of breeders and geneticists. The program has released more than 30 Acala 1517 cotton (Gossypium hir- art (1977). In South Carolina, genetic gain in yield imsutum L.) cultivars and numerous germplasm lines known for high fiber provements was 10.5 kg ha Ϫ1 yr Ϫ1 for commercial cultiquality and Verticillium wilt (caused by Verticillium dahliae Kleb.) tolvars and 15.1 kg ha Ϫ1 yr Ϫ1 for Pee Dee germplasm lines erance that have made substantial contributions to cotton breeding in from 1945 to 1978 (Culp and Green, 1992). For Califorthe USA. The present project was initiated in 2003 to evaluate the genia Acala cotton, Bassett and Hyer (1985) estimated netic improvement of Acala 1517 cultivars and lines released over the the genetic gain of 8.0 kg ha Ϫ1 yr Ϫ1 from 1930 to 1980. past 75 yr in yield, boll size, seed index, lint percentage, fiber length, fiber In the Mississippi Delta, genetic gain in lint yield imstrength, and micronaire. Their genetic divergence was also estimated by provements averaged 9.1 to 10.2 kg ha Ϫ1 yr Ϫ1 from 1922 simple sequence repeat (SSR) markers. On the basis of the data availto 1966 and 8.5 to 9.5 kg ha Ϫ1 yr Ϫ1 from 1910 to 1978; able from annual yield trials, lint yield and lint percentage in Acala however, the rate was shown to be decreased when a 1517 cotton have steadily increased since the 1930s, while boll size and seed index have gradually decreased since the 1960s. Fiber strength has longer period was included in the analysis: 5.4 kg ha Ϫ1 been enhanced since the 1960s, which has been accompanied by steady yr Ϫ1 from 1938 to 1993 and 4.7 kg ha Ϫ1 yr Ϫ1 from 1938 to increase in micronaire. However, fiber length in Acala 1517 cultivars 1999. The genetic gain was further decreased to 3.5 kg tended to shorten from 31.0 to 30.0 mm from 1960 to 1990, whereas ha Ϫ1 yr Ϫ1 from 1983 to 1999 (Bridge et al., 1971; Bridge newly released Acala 1517 cultivars (Acala 1517-95, 1517-99, 1517-02, and Meredith, 1983; Meredith 2000). The slowed ge-1517-03, and 1517-04) have fiber greater than 30.5 mm. Genetic distance netic gain in cotton yield improvement was thought to among Acala 1517 genotypes ranged from 0.06 to 0.38 with an average be due to narrow genetic base of upland cotton and of 0.18 on the basis of 189 SSR marker alleles, indicating a substantial repeated use of a few upland cotton germplasm in major genetic diversity among Acala 1517 cotton germplasm. Divergent germcommercial cotton breeding programs (May et al., 1995; plasm introgression in the program has contributed to genetic diver-Meredith, 2000; Lewis, 2001). sity of Acala cotton germplasm and continuous genetic gain in Acala cotton cultivar improvement.

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currently the only feasible means for production of haploids in cotton is via semigamy. Haploids produced by semigamy can be used to facilitate selectionSemigamy is a type of facultative apomixis in which in cotton (Gossypium barbadense L.) breeding and gene mapping. The the male sperm nucleus does not fuse with the egg objectives of the present study were to detail the genetic mechanism of nucleus after penetrating the egg in the embryo sac. Subsemigamy and its relationship with reduction in chlorophyll content. Pima 57-4, a semigametic line isolated as a doubled haploid (DH) sequent development can give rise to an embryo confrom Pima S-1, produced 46 to 50% haploid progeny when selfed.taining haploid chimeral tissues of paternal and mater-Compared with its natural isogenic line S-1, 57-4 had lower leaf chloronal origins. In cotton, the semigametic phenomena was phyll content, photosynthetic rate (Pn), and boll weight, and had first reported by Turcotte and Feaster (1963), who dehigher lint percentage, seed index, and shorter and coarser fiber.veloped the Pima line 57-4 that produced haploid seeds Three segregating populations (F 2 and two BC 1 F 1 ) from a cross of at a high frequency. The semigametic line was a DH 57-4 ϫ S-1 were progeny tested to investigate the genetic basis of from a natural haploid mutant found in a field of Pima semigamy. Haploid production in F 1 , F 2 , and backcrosses were also S-1. In two presentations, Turcotte and Feaster (1974, recorded. The results verified that semigamy in cotton is controlled 1975) indicated that semigamy in Pima appeared to be by one gene, previously designated Se. The gene functions sporophytcontrolled by one dominant gene (Se). Despite wide ically and gametophytically, resulting in incomplete dominance nature. A generation-mean analysis for the cross of 57-4 ϫ S-1 identified acceptance by the cotton research community (Kohel one gene responsible for the parental differences in chlorophyll conand Lewis , 1984;Smith and Cothren, 1999), no compretent. Consistent with the difference between the two parental isogenic hensive genetic data have been published. Semigametic lines, semigametic F 2.3 lines had significantly lower chlorophyll contentlines can produce 30 to 60% haploids when self pollithan nonsemigametic F 2.3 lines, an observation that was confirmed by nated, and about 0.7 to 1.0% androgenic haploids when a significant association between haploid production and chlorophyll used as female parents in crosses with normal nonsemicontent. The Se gene and the gene for reduced chlorophyll content gametic cottons (Chaudhari, 1978(Chaudhari, , 1979; Turcotte and could be either the same or closely linked. The elucidation of the Feaster, 1967). A unique feature of semigamy is that relationship between semigamy and fiber development will help charthe inheritance of the gene is conveyed by both male acterize the molecular function of the semigametic gene. and female gametes, but expression of the trait in terms of haploid production occurs only in the female parent.

The southern root-knot nematode (RKN) [Meloidogyne incognita (Kofoid and White) Chitwood) RKN] is one of the most destructive pests in the Cotton Belt of the U.S. Developing and employing resistant cultivars is the most economical and efficient method for RKN management. This greenhouse investigation was conducted to understand the quantitative genetic basis of RKN resistance in the major RKN resistance sources using a nine-parent diallel analysis based on improved RKN evaluation techniques. The selected genotypes consisted of three ÔAubrun 623 RNRÕ (here RNR ¼ Root-knot Nematode Resistance)-derived resistant lines, one moderately resistant cultivar ÔAcala Nem-XÕ, and five susceptible cultivars. Comparison between F 1 and their parents in galling index revealed that the RKN resistance is partially dominant. The general combining ability was more important than the specific combining ability for RKN resistance. The estimates for broad-sense and narrow-sense heritabilities on galling index were 0.82 and 0.65, respectively, indicating that RKN resistance in the Auburn and Nem-X resistance sources is largely controlled by genetic and additive effects. The minimum number of genes for RKN resistance was estimated to be two. Therefore, under reliable and uniform RKN inoculation and infestation conditions, single plant selection should be efficient in transferring Auburn RKN resistance into elite genetic backgrounds.The southern root-knot nematode (RKN) [Meloidogyne incognita (Kofoid and White) Chitwood)] race 3 is one of the major threats to US cotton production (Blasingame and Patel 2005). Developing and using resistant cultivars represents one of the most efficient and economical strategies in RKN management. In cotton, many moderately RKN resistant germplasms have been identified and a highly RKN resistant germplasm line, ÔAubrun 623 RNRÕ (here RNR ¼ Root-knot Nematode Resistance), was released by combining moderate resistance from both of its parents (ÔClevewilt 6Õ and ÔWild Mexico Jack JonesÕ) (Shepherd 1974c). A significant research effort has since been made to investigate the genetics and mechanisms of this resistance and to develop RKN resistant cultivars (McPherson et al. 1995, Colyer et al. 2000. The high level of RKN resistance from ÔAubrun 623 RNRÕ was transferred into various genetic backgrounds to develop a series of new RKN resistant lines, such as ÔAuburn 634 RNRÕ (in ÔAuburn 56Õ background) (Shepherd, 1982), M-240 RNR, and M-315 RNR (both in DP61 background) (Shepherd et al. 1996). The resistance from ÔClevewilt 6Õ was also used to develop the moderately resistant cultivars ÔLA 434 RKRÕ, ÔST LA 887Õ, and ÔPM 1560Õ. A moderately RKN resistant cotton cultivar ÔAcala Nem-XÕ was released with its resistance source unknown (Ogallo et al. 1999, Robinson et al. 1999). However, the high-RKN resistance level from the Auburn source has never been transferred into any commercial cotton cultivar.The inheritance of RKN resistance in ÔAubrun 623 RNRÕ and its derived resistant lines has been investigated since the 19...

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