Astragalus mokiacensis Gray (Fabaceae), and Astragalus preussii Gray var. laxifolorus Gray are taxa endemic to the Mojave Desert that may be hybrids between two different sections of this genus. A. mokiacensis is endemic to northeastern Clark County, Nevada and northwestern Mohave County, Arizona. A. preussii var. laxiflorus is endemic to the Lake Mead region of eastern Clark County, Nevada. These species are sympatric around Lake Mead, however, A. preussii var. laxiflorus is restricted to gypsum soils and A. mokiacensis can be found on a variety of soil types. Two prevalent theories exist on the taxonomic relationships of A. mokiacensis and A. preussii var. laxiflorus: either they are related to one of several varieties of A. lentiginosus (Section Diphysi) or they are related to A. preussii var. preussii (Section Preussiani). Alternatively, they could be recombinant hybrids between the two species. The relationships between A. mokiacensis, A. preussii var. laxiflorus, A. preussii var. preussii, A. lentiginosus var. ambiguus, A. lentiginosus var. palans, A. lentiginosus var. yuccanus will be investigated based on analyses of the internal transcribed spacer region (ITS) and morphological data.

Key words: Astragalus mokiacensis, Astragalus preussii var. laxiflorus, ITS, Mojave Desert, Nevada, recombinant speciation