The cost of realized sexual reproduction: Assessing patterns of reproductive allocation and sporophyte abortion in a desert moss

The desert moss Syntrichia caninervis exhibits one of the most skewed sex r atios in the plant kingdom, with female individuals far outnumbering male i ndividuals (exceeding 14 female :1 male). The "cost of sex hypothesis" deri ves from allocational theory and predicts that the sex which is most expens ive should be the rarer sex. This hypothesis, which. as considered here rep resents the realized cost of sexual reproduction, is contingent upon two as sumptions that are explored: (1) that male sex expression is more expensive than female sex expression, and (2) that sexual reproduction is resource l imited. Using inflorescence biomass and discounting sperm, male sex express ion was found to be in the neighborhood of one order of magnitude more expe nsive than female sex expression, and this difference is reflected in highe r numbers of gametangia per male inflorescence, presence of paraphyses in m ale inflorescences, and a much longer developmental time for male infloresc ences. The realized cost of female reproduction from two communities domina ted by S, caninervis was found to be lower than the realized cost of male s exual reproduction. Resource-limited reproduction was assessed by determini ng the frequency of sporophyte abortion, the age distribution of sporophyte abortions, and patterns of sporophyte abortion that may be density depende nt. Among ten sexually reproducing populations, abortive sporophytes occurr ed at a frequency of 0.64. Abortive sporophytes averaged 8% the mass of mat ure sporophytes, and cohort sporophytes from the same individual female wer e found to abort in a density-dependent pattern. We conclude that the two a ssumptions, upon which the cost of sex hypothesis depends, are supported.