Why be a honeyless honey mesquite? Reproduction and mating system of nectarful and nectarless individuals

Populations of Prosopis glandulosa var. torreyana in the Chihuahuan desert have a fixed dimorphic system of nectar production in which half the indivi duals produce nectar (are nectarful) and the other half are nectarless. We analyzed the impact of nectar production on different estimates of fitness, comparing nectarful against nectarless individuals in size, mating system, seed traits, and fruit set in a 1-ha scrubland. Of the reproductive indivi duals (358), 46% were nectarful and 54% were nectarless. Neither tree size nor flowering phenology differed between nectar morphs. Fixation indices (F ) for both progeny (F = -0.2) and adults (F = -0.45) were negative, and hig h heterozygosities were found in adults and progeny (H = 0.45). No differen ces were found between nectar morphs for F, H, and single (t(s) = 1.1) and multilocus (t(m) = 1.03) outcrossing rates. Controlled pollinations showed differences between selfing and control treatments with no differences betw een nectar morphs. Nectarless individuals produced significantly more polle n grains than did nectar producers, but all other measured floral traits sh owed no differences. Nectarful trees were visited by pollinators 21 times m ore often and had a significantly higher overall fruit set than did nectarl ess trees. No differences between nectar morphs in seed mass or in percenta ge seed germination were found, but heavier seeds tended to have higher het erozygosities. Both morphs had similar success as females, but nectarless t rees had similar to 7% higher male function. We discuss three possible scen arios for the evolution of the fixed dimorphism in nectar production, two i nvolving unstable phases (substitution of one morph by the other, and evolu tion towards dioecy) and one stable scenario (maintenance of the dimorphic system).