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The cost of realized sexual reproduction: assessing patterns of reproductive allocation and sporophyte abortion in a desert moss¹

² Department of Biological Sciences, University of Nevada, 4505 Maryland Parkway, Box 454004, Las Vegas, Nevada 89154-4004 USA; ³ University Herbarium, Jepson Herbarium, and Department of Integrative Biology, 1001 Valley Life Sciences Bld., #2465, University of California, Berkeley, California, 94720-2465 USA; and ⁴ T.H. Morgan School of Biological Sciences, 101 Morgan Bld., University of Kentucky, Lexington, Kentucky 40506-0225 USA

The desert moss Syntrichia caninervis exhibits one of the most skewed sex ratios in the plant kingdom, with female individuals far outnumbering male individuals (exceeding 14:1). The "cost of sex hypothesis" derives from allocational theory and predicts that the sex which is most expensive should be the rarer sex. This hypothesis, which, as considered here represents the realized cost of sexual reproduction, is contingent upon two assumptions that are explored: (1) that male sex expression is more expensive than female sex expression, and (2) that sexual reproduction is resource limited. Using inflorescence biomass and discounting sperm, male sex expression was found to be in the neighborhood of one order of magnitude more expensive than female sex expression, and this difference is reflected in higher numbers of gametangia per male inflorescence, presence of paraphyses in male inflorescences, and a much longer developmental time for male inflorescences. The realized cost of female reproduction from two communities dominated by S. caninervis was found to be lower than the realized cost of male sexual reproduction. Resource-limited reproduction was assessed by determining the frequency of sporophyte abortion, the age distribution of sporophyte abortions, and patterns of sporophyte abortion that may be density dependent. Among ten sexually reproducing populations, abortive sporophytes occurred at a frequency of 0.64. Abortive sporophytes averaged 8% the mass of mature sporophytes, and cohort sporophytes from the same individual female were found to abort in a density-dependent pattern. We conclude that the two assumptions, upon which the cost of sex hypothesis depends, are supported.

Key Words: allocation • bryophyte • cost of sex • cryptogamic crust • desert • haploid dioecy • sex ratios • sporophyte abortion • Syntrichia caninervis.

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