Hexalectris

Abstract: Hexalectris is a genus of mycoheterotrophic orchids found in the southwestern United States and Mexico, and some species in the genus are considered of conservation concern. Details on the evolutionary relationships between Hexalectris and symbiotic fungi are known, but relatively little is understood about the ecological factors that affect geographical distribution and seasonal variation in populations. Over a 12-year period, I (with volunteers and student assistants) studied the relationship between Hexalectris numbers and annual rainfall, and between documented orchid locations and soil–geological characteristics in Dallas County, Texas. We found a strong relationship between Hexalectris census numbers and the previous year's total rainfall. Targeted searches informed by knowledge of soil type helped to identify nine Hexalectris populations in Dallas County and a range expansion in H. grandiflora, a species previously unknown outside of western Texas. Hexalectris diversity correlates with preserve area, demonstrating that a larger preserve could help conserve more Hexalectris species. Overlapping soil surveys, geological data, and known Hexalectris locations in Texas helped to identify 64 counties with soil and geological conditions associated with Hexalectris. Many of the counties have no records of Hexalectris, showing the potential need for and value of targeted searches during wetter years to aid in identifying new populations to help fill in Hexalectris distributions and direct future conservation efforts.

Abstract: New combinations and names are proposed for species formerly placed in three genera of subtribe Bletiinae. Based on the latest phylogenetic analyses of the subtribe in which species of Basiphyllaea and Hexalectris were embedded among the species of Bletia, twelve new combinations and a new name are proposed in the last.

Abstract: Mycoheterotrophic species have abandoned an autotrophic lifestyle and obtain carbon exclusively from mycorrhizal fungi. Although these species have evolved independently in many plant families, such events have occurred most often in the Orchidaceae, resulting in the highest concentration of these species in the tracheophytes. Studies of mycoheterotrophic species’ mycobionts have generally revealed extreme levels of mycorrhizal specialization, suggesting that this system is ideal for studying the evolution of mycorrhizal associations. However, these studies have often investigated single or few, often unrelated, species without consideration of their phylogenetic relationships. Herein, we present the first investigation of the mycorrhizal associates of all species of a well-characterized orchid genus comprised exclusively of mycoheterotrophic species. With the employment of molecular phylogenetic methods, we identify the fungal associates of each of nine Hexalectris species from 134 individuals and 42 populations. We report that Hexalectris warnockii associates exclusively with members of the Thelephoraceae, H. brevicaulis and H. grandiflora associate with members of the Russulaceae and Sebacinaceae subgroup A, while each member of the H. spicata species complex associates primarily with unique sets of Sebacinaceae subgroup A clades. These results are consistent with other studies of mycorrhizal specificity within mycoheterotrophic plants in that they suggest strong selection within divergent lineages for unique associations with narrow clades of mycorrhizal fungi. Our results also suggest that mycorrhizal associations are a rapidly evolving characteristic in the H. spicata complex.