BREEDING SYSTEM IN LIMNANTHES ALBA: SEVERAL ALTERNATIVE MEASURES

Seven populations of Limnanthes alba were described for their rates of outbreeding, amount of genetic variability, and response to enforced selfing in terms of inbreeding depression. Outcrossing rates, estimated by using two genetic marker loci, varied between 43 and 97%. Heterozygosity level was found to be in the range of 12 and 27 percent, whereas percent polymorphic loci varied twofold (between 29 and 57%) among populations. Inbreeding depression was found to be significant in four of seven populations for number of flowers per plant, plant height, and seed set per flower. There was no association between observed patterns of inbreeding response and predictions from the genetic variation parameters, so that variations in outbreeding rates and consequences need a detailed quantitative verification. In populations showing no inbreeding depression, autofertility seemed to increase with inbreeding (presumably accompanied by natural selection) and to be under genetic control. This is relevant to the hypothesis on the origin of inbreeders under conditions lacking pollinators, e.g., colonization of marginal areas. THE GENUS Limnanthes, first treated in a monograph by Mason (1952), has been a subject of several evolutionary studies in recent years. Ornduff (1971), Arroyo (1975), and Jain (1976b) discussed the morphological and biosystematic diversity of nearly 18 known taxa in this genus of endemic annuals occurring in vernal pools and moist meadows in California, Oregon, and Vancouver Island (with a single species L. macounii). Its attractive white and yellow flowers are pollinated by several native bees (e.g., Andrena, Panur-ginus) and the honeybee. Mason (1952) and Arroyo (1975) observed several degrees of protandry (in terms of number of days) in different populations of L. alba, an outbreeder, and various degrees of cleistogamy in an inbreeder, L. floccosa. Protandry, cleistogamy, and several morphological traits such as petal size and shape of corolla were associated with the breeding system. Arroyo (1975) also attempted to correlate genetic variation as measured by electrophoretic assays with the breeding system as "estimated" by protandry and autofertility, i.e., ability to set seed in a pollinator-free environment. Her main conclusion was that inbreeder and outbreeder species have similar genetic structures. Our own genetic surveys of variation (Brown et al., unpublished), however, show that L. floccosa is much less variable within populations than L. alba. It should be emphasized that comparisons of genetic structures of related species ' Received for publication 25 April 1977; revision accepted 27 June 1977. The author is grateful to Charles R. Brown and Holly Hauptli for their technical help and useful suggestions. require a careful quantitative description of the breeding system in nature and evaluation of the significance of descriptors of genetic variation (Jain, 1976a). For example, how much do different populations of L. alba vary in the degree of random outcrossing? How much heterozygosity is maintained at the polymorphic loci? If observed levels of outbreeding and heterozygosis are somehow selectively maintained in relation to the adaptive role of genetic homeostasis, say through overdominance (Lerner, 1954) as the factor underlying an excess of heterozygotes, would inbreeding result in depression correlated with overdominance? Finally, if inbreeders have evolved from outbreeders under stress environments presumably through increased evolution toward autogamy (and autofertility), are genetic control of autofertility and selection response demonstrable in controlled studies? The first two of these questions require comparative studies of two or more related species for quantitative determination of their breeding system and genetic variation, whereas the last two can be answered from studies on the effects of inbreeding and artificial selection in an outbreeder. Here, I shall briefly describe the breeding system, genetic parameters of variability, and autofertility under inbreeding in seven populations of L. alba chosen to represent a range of estimated outbreeding rates; the results of selection experiments will be reported in another paper. MATERIALS AND METHODS-The seven populations of L. alba used were sampled in the spring of 1973 and 1974. Individual plants were