Introgression as a Spatial Phenomenon

Abstract

Biologists are devoting increasing attention to introgression, which is relevant to several topics of current concern. Since it is inherently a spatial phenomenon, introgression should be especially interesting to geographers. Hybridization, followed by introgression, occurs where two species (or ecotypes) meet, often after migration of one or both brings the two into proximity. If new genotypes created by hybridization and introgression are successful, they are likely to migrate further, while also increasing in frequency. Consequently, spatiotemporal analysis is usually necessary to properly interpret possible cases of introgression. Hypotheses are best formulated and tested by modifying the standard spatial introgression models to allow for hybrid fitness, temporal change, and real world habitat mosaics. In contrast, analyses that presume equilibrium population genetics, or that treat each taxonomic species as reproductively inviolate, inevitably lead to mistaken conclusions. A model is outlined showing how hybridization between a crop and wild relative may increase invasiveness, particularly in productive habitats. [

Biogeography and Genetic Diversity of Pearl Millet (Pennisetum glaucum) from Sahelian Africa

“…In regions of crop domestication that have not undergone a rapid rate of agricultural development, the chance for continued gene flow, hybridization, and introgression between various crop subspecies and species exists and is well documented (Ellstrand, Prentice, and Hancock 1999;Jarvis and Hodgkin 1999;Elias, Rival, and McKey 2000;Blumler 2003;Haygood, Ives, and Andow 2003;Papa and Gepts 2004;Zizumbo-Villarreal et al 2005;Duputié et al 2007). …”

“…When these barriers do not prohibit gene flow among populations, hybrid progeny result that maintain specific wild and domesticated alleles. In regions of crop domestication where agriculture is significant, introgression can be common between wild and domesticated material (Ellstrand, Prentice, and Hancock 1999;Blumler 2003;Dubcovsky and Dvorak 2007). The maintenance of such material and the subsequent increased levels of genetic diversity in these regions, however, are also dependent on species biogeography and agricultural management practices by farmers (Elias, Rival, and McKey 2000;Blumler 2003;Couturon et al 2003).…”

“…Patterns of molecular and morphological diversity can be dissimilar within a particular species, leading us to question conclusions about the impact of human-mediated, adaptive selection on genomic diversity (Wang et al 1999). Recent technological developments and the use of various molecular methodologies have enhanced our ability to comprehend genetic differentiation within and among populations, and biogeography, in particular, can make a significant contribution in this field of inquiry (Blumler 2003;Young 2003). Examination of pearl millet differentiation at the population level was aided by microsatellites or simple sequence repeats (SSRs).…”

“…The specific consequences of recurrent hybridization among various intraspecific populations are (1) possible reduction or loss of genetic diversity in wild populations (e.g., beans; ZizumboVillarreal et al 2005), (2) reduction or loss of domesticated traits in managed populations (e.g., cassava; Elias, Rival, and McKey 2000), and (3) the creation of intermediate types that can become fixed and stable within the agroecosystem (e.g., pearl millet; Couturon et al 2003;Miura and Terauchi 2005). The diversity present in African pearl millet landraces might be a function of gene flow not only among domesticated types but also among wild types and weedy hybrids that germinate spontaneously throughout their native range and might reduce the risks of management under extreme environmental heterogeneity by providing agroecological functionality and benefit to farmers (Blumler 2003;Hooper et al 2005). …”

Hybridization and the colonization of novel habitats by annual sunflowers

“…Alternatively, new hybrid gene combinations may become established through allopolyploidy, clonal or asexual reproduction, selfing, or diploid hybrid speciation (Barton 2001). Regardless of mechanism, if the hybrid gene combinations enhance growth and competitive ability in novel environments, invasive success may be increased (Harlan and De Wet 1963;Lewontin and Birch 1966;Small 1984;Stebbins 1985;Abbott 1992;Galatowitsch et al 1999;Kim and Rieseberg 1999;Ellstrand and Schierenbeck 2000;Blumler 2003). Of course, in many instances hybridization will have no impact on invasiveness.…”