The mystery of hybridization between the federally threatened Lespedeza leptostachya Englem. and its co-occurring common congener L. capitata Michx. (Fabaceae)

Background/Question/Methods
Natural hybridization is common in the genus Lespedeza (Clewell 1966) and has been identified as a possible threat to the federally threatened Lespedeza leptostachya. Lespedeza leptostachya appears to be hybridizing with its co-occurring common congener L. capitata.The presence of hybrids is enigmatic because L. leptostachya primarily reproduces via cleistogamous flowers (CL) and produces few chasmogamous flowers (CH) (Sather 1988) which would allow for cross pollination, let alone hybridization. Anecdotal evidence indicates that L. leptostachya populations in Minnesota, where most hybrids are reported, seem to produce more CH flowers (MN DNR personal correspondence) providing a possible mechanism for hybridization. We used a two fold approach to test the putative hybridization between L. leptostachya and L. capitata.First, we conducted field studies in eight Minnesota populations of L. leptsostachya and L. capitata to: 1. Measure morphological differences among parents and the putative hybrids; 2. Count the ratio of CH to CL flowers; and 3. Record visitors. Second, using DNA extracted from leaf tissue of L. leptostachya and L. capitata, we identified species specific differences in the trnL-trnF gene region of cpDNA and the ITS gene region of nDNA to compare the putative hybrid DNA to its parents. We hypothesized that 1. Hybrids will exhibit intermediate traits compared to the parent species; 2. CH to CL flower ratios will be higher in the presence of hybrids; 3. Potential pollinators will visit both L. leptostachya and L. capitata. 4. Hybrids will display cpDNA sequences similar to only one species and nDNA from both species.

Results/Conclusions
We found that putative hybrids showed intermediate traits to parents for 15 of 19 morphological characters, with 11 being significantly different (p<0.05). Hybrids also displayed two novel traits: more branching and shorter panicles than both parents (p<0.05). A discriminate analysis clearly grouped the hybrids as intermediate to both parents. The CH to CL flower ratio was significantly greater in the presence of hybrids (p<0.05), and Satyrium sp. (Lepidoptera) visited both parent species; indicating a mechanism for cross pollination and hybrid formation. The presence of hybrids was confirmed with genetic studies which showed that hybrids exhibited cpDNA identical to L. leptostachya and nDNA identical to both parents. Our studies support that hybridization is occurring in the Minnesota populations of L. leptsostachya. Further investigations into the impact of the hybrids on L. leptostachya in these populations and others are necessary to prevent the loss of this federally threatened species.