The tree roots at an ancestral genome size of approximately 1x = 0.2 pg. Arabidopsis thaliana (1C = 0.16 pg; approximately 157 Mbp) has the smallest genome size in Brassicaceae studied here and apparently represents an evolutionary decrease in genome size. Two other branches that represent probable evolutionary decreases in genome size terminate in Lepidium virginicum and Brassica rapa. Branches in the phylogenetic tree that represent probable evolutionary increases in genome size terminate in Arabidopsis halleri, A. lyrata, Arabis hirsuta, Capsella rubella, Caulanthus heterophyllus, Crucihimalaya, Lepidium sativum, Sisymbrium and Thlaspi arvense. Branches within one clade containing Brassica were identified that represent two ancient ploidy events (2x to 4x and 4x to 6x) that were predicted from published comparative mapping studies.

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The DNA sequence phylogeny splits Sorghum into two lineages, one comprising the 2n = 10 species with large genomes and their polyploid relatives, and the other with the 2n = 20, 40 species with relatively small genomes. An apparent phylogenetic reduction in genome size has occurred in the 2n = 10 lineage. Genome size evolution in the genus Sorghum apparently did not involve a 'one way ticket to genomic obesity' as has been proposed for the grasses.

Gene flow between crops and their weedy or wild relatives can be problematic in modern agricultural systems, especially if it endows novel adaptive genes that confer tolerance to abiotic and biotic stresses. Alternatively, gene flow from weedy relatives to domesticated crops may facilitate ferality through introgression of weedy characteristics in the progeny. Cultivated sorghum (Sorghum bicolor), is particularly vulnerable to the risks associated with gene flow to several weedy relatives, johnsongrass (S. halepense), shattercane (S. bicolor ssp. drummondii) and columbusgrass (S. almum). Johnsongrass and shattercane are common weeds in many sorghum production areas around the world. Sorghum varieties with adaptive traits developed through conventional breeding or novel transgenesis pose agronomic and ecological risks if transferred into weedy/wild relatives. Knowledge of the nature and characteristics of gene flow among different sorghum species is scarce, and existing knowledge is scattered. Here, we review current knowledge of gene flow between cultivated sorghum and its weedy and wild relatives. We further discuss potential avenues for addressing gene flow through genetic, molecular, and field level containment, mitigation and management strategies to facilitate successful deployment of novel traits in this economically important crop species.

sists of seven Asian, Australian, and central American species (Lazarides et al., 1991). Ten species that occur in Sorghum [Sorghum bicolor (L.) Moench] breeders have long recnorthern Australia comprise the Stiposorghum section ognized the importance of exotic germplasm and noncultivated sorghum races as sources of valuable genes for genetic improvement. The (Lazarides et al., 1991). genus Sorghum consists of 25 species classified as five sections: Eu-Sorghum breeders have long recognized the imporsorghum, Chaetosorghum, Heterosorghum, Para-sorghum, and Stipotance of exotic germplasm (Duncan et al., 1991). Nonsorghum. Species outside the Eu-sorghum section are sources of imporcultivated sorghum races have been extensively used as tant genes for sorghum improvement, including those for insect and sources of genes for sorghum improvement (Rosenow disease resistance, but these have not been used because of the failure and Dahlberg, 2000). However, no species outside the of these species to cross with sorghum. An understanding of the bioeu-sorghum section have been utilized because of strong logical nature of the incompatibility system(s) that prevent hybridizareproductive barriers (Garber, 1950; Schertz and Dalton, tion and/or seed development is necessary for the successful hybridi-1980; Doggett, 1988). Resistance to major insects and diszation and introgression between sorghum and divergent Sorghum eases, for example, midge [Stenodiplosis (Contarinia) sorspecies. The objectives of this study were to determine the reason(s) for reproductive isolation between Sorghum species. The current study ghicola (Coquillett)] and downy mildew [caused by Peutilized 14 alien Sorghum species and established that pollen-pistil ronosclerospora sorghi (Weston and Uppal) Shaw], that incompatibilities are the primary reasons that hybrids with sorghum attack sorghum has been found in species of the Chaetoare not obtained. The alien pollen tubes showed major inhibition of sorghum, Heterosorghum, Para-sorghum, and Stiposorgrowth in sorghum pistils and seldom grew beyond the stigma. Pollen ghum sections (Franzmann and Hardy, 1996; Sharma tubes of only three species grew into the ovary of sorghum. Fertiliza-