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The genus Carya includes diploid (2n=2x=32) and tetraploid (2n=4x=64) species native to forests in the United States,Mexico and Asia.The greatest number and diversity of species are found in the U.S.,where six diploid and five tetraploid species representing two sections (Carya and Apocarya) are found.The species often occur in mixed populations with frequent interspecific hybridization which has been confirmed with plastid and nuclear molecular markers.Interspecific hybrids have been described between diploid species pairs,tetraploid species pairs and between diploid and tetraploid species and may impact regional adaptation.Ploidy level as well as geographic origin of populations under evaluation greatly impact performance and have implications for rootstock and scion development in pecan (Carya illinoinensis) culture.Only diploid species are found in Mexico,including three (Carya illinoinensis,C.ovata,C.myristiciformis) also found in the U.S.One endemic (C.palmeri) is found only in Mexico.Mexican Carya populations are sympatric and show molecular genetic evidence of hybridization.Several species (C.cathayensis,C.dabieshanensis,C.tonkinensis,C.hunanensis,and C.kweichowensis),all putative diploids of section Sinocarya,are distributed in isolated,primarily single species populations from India,Laos and Vietnam to eastern China.Carya poilanei,reported only from Vietnam and Laos and possibly already extinct,has characteristics that unite it with extant members of section Apocarya in the U.S.and Mexico,and with fossil species of Carya found in Europe.The disputed taxon Carya sinensis (=Annamocarya sinensis) is found in limited stands in Vietnam and China and is considered critically endangered.Significant international attention is focused on pecan production.The export of this wind-pollinated tree species to other countries with native Carya species raises questions of the impact this may have on the population dynamics of species we know little about.Targeted collections,improved molecular genetic tools for their characterization,and careful attention to phytosanitary procedures involved in their exchange are needed in order to strategically plan for the conservation and utilization of this globally important genus of trees.