INBREEDING DEPRESSION IN PARTIALLY SELF-FERTILIZING DECODON VERTICILLATUS (LYTHRACEAE) - POPULATION-GENETIC AND EXPERIMENTAL ANALYSES

Inbreeding depression is a major selective force favoring outcrossing in flowering plants. Some self-fertilization, however, should weaken t he harmful effects of inbreeding by exposing genetic load to selection . This study examines the maintenance of inbreeding depression in part ially self-fertilizing populations of the long-lived, herbaceous wetla nd plant, Decodon verticillatus (L.) Ell. (Lythraceae). Estimates from ten populations indicate that 30% of offspring are produced through s elf-fertilization. Population-genetic estimates of inbreeding depressi on (delta = 1 - relative mean fitness of selfed progeny) based on chan ges in the inbreeding coefficient for the same ten populations were un iformly high, ranging from 0.49 to 1.79 and averaging 1.11 +/- 0.29 SE . Although confidence intervals of individual population estimates wer e large, estimates were significantly greater than 0 in six population s and greater than 0.5 in four. Inbreeding depression was also estimat ed by comparing growth, survival, and flowering of experimentally self ed and outcrossed offspring from two of these populations in a l-yr gl asshouse experiment involving three density regimes; after which offsp ring were transplanted into garden arrays and two field sites and moni tored for two consecutive growing seasons. Overall ($) over cap delta for survival averaged 0.27 +/- 0.01 in the glasshouse, 0.33 +/- 0.04 i n the garden, and 0.46 +/- 0.04 in the field. The glasshouse experimen t also revealed strong inbreeding depression for growth variables, esp ecially above-soil dry weight (($) over cap delta = 0.42 +/- 0.03). Th e fitness consequences of inbreeding depression for these growth varia bles approximately doubles if survival to maturity is determined by se vere truncation selection. Despite substantial selfing, inbreeding dep ression appears to be a major selective force favoring the maintenance of outcrossing in D. verticillatus.