The amounts of inbreeding depression upon selfing and of heterosis upon outcrossing determine the strength of selection on the selfing rate in a population when this evolves polygenically by small steps. Genetic models are constructed which allow inbreeding depression to change with the mean selfing rate in a population by incorporating both mutation to recessive and partially dominant lethal and sublethal alleles at many loci and mutation in quantitative characters under stabilizing selection. The models help to explain observations of high inbreeding depression (> 50%) upon selfing in primarily outcrossing populations, as well as considerable heterosis upon outcrossing in primarily selfing populations. Predominant selfing and predominant outcrossing are found to be alternative stable states of the mating system in most plant populations. Which of these stable states a species approaches depends on the history of its population structure and the magnitude of effect of genes influencing the selfing rate.

The evolution of self-fertilization and inbreeding depression in plants. i. genetic models

Dichogamy is the separation of the presentation of pollen and stigmas in time within a plant. It is a common but neglected feature of outcrossing angiosperms. Dichogamy has been almost universally interpreted as an outcrossing mechanism, but many dichogamous species are also self-incompatible (and sometimes also herkogamous and/or with unisexual flowers). In outcrossing species, there is almost invariably a clash between selection to place pollen and stigmas in similar positions for effective pollination and selection to keep the androecia and gynoecia apart to avoid interference between them. We suggest that the separation of pollen and stigmas acts in general to reduce this self-interference and it often also reduces self-fertilisation. Mechanisms preventing self-fertilisation primarily increase maternal fitness, whereas mechanisms avoiding self-interference primarily promote paternal fitness. Five independent ways of subdividing dichogamy are recognised: protandry or protogyny; intrafloral or ...

https://www.tandfonline.com/doi/pdf/10.1080/0028825X.1986.10409726

The avoidance of interference between the presentation of pollen and stigmas in angiosperms II. Herkogamy

A bimodal distribution of outcrossing rates was observed for natural plant populations, with more primarily selfing and primarily outcrossing species, and fewer species with intermediate outcrossing rate than expected by chance. We suggest that this distribution results from selection for the maintenance of outcrossing in historically large, outcrossing populations with substantial inbreeding depression, and from selection for selfing when increased inbreeding, due to pollinator failure or population bottlenecks, reduces the level of inbreeding depression. Few species or populations are fixed at complete selfing or complete outcrossing. A low level of selfing in primarily outcrossing species is unlikely to be selectively advantageous, but will not reduce inbreeding depression to the level where selfing is selectively favored, particularly if accompanied by reproductive compensation. Similarly, occasional outcrossing in primarily selfing species is unlikely to regularly provide sufficient heterosis to maintain selection for outcrossing through individual selection. Genetic, morphological and ecological constraints may limit the potential for outcrossing rates in selfers to be reduced below some minimum level.

The evolution of self-fertilization and inbreeding depression in plants. ii. empirical observations.

This survey emphasizes the possibilities of isozymes as marker genes in solving a variety of problems important to population geneticists, evolutionists, systematists, and plant breeders.

Isozymes in plant biology

Phenotypic plasticity is an important element in the response repertoire of plants (25). It may be manifested as changes in organ (e.g. leaf) morphology as well as in patterns of biomass distribution. Morphological plasticity throughout growth is possible because plant development is modular in form (77, 184, 185). Growth results from the reiteration of basic morphological subunits produced by meristems. Because meristems may develop into either reproductive or vegetative structures, patterns of biomass distribution reflect developmental decisions (121, 172, 180). In this essay, we review data suggesting that at least in certain cases, architectural constraints affect the range of morphological plasticity that can be expressed. Because carbon is frequently viewed as a critical currency of allocation (76, 165), we focus on how these constraints influence assimilate (i.e. carbon) production and utilization. We postulate that when these constraints are present, plants consist not only of morphological subunits (185) but, as Adams (4) first suggested, of physiological subunits as well. We call these integrated physiological units (IPUs) and propose that they are made up of identifiable arrays of morphological subunits that together function as relatively autonomous structures with respect to the assimilation, distribution, and utilization of carbon. If such units of physiological integration exist,

Morphogenetic constraints on patterns of carbon distribution in plants

Genotypic differences among forms of Lupinus nanus were established for the reproductive traits of flowering time, flower size, pollen production, ovule production, length of anthers, length of stigmatic hairs, coherence between keel margins, blue reflectance from corollas, honey guide marking, and autofertility These forms outcrossed at significantly different rates when exposed to the same pollination environment. Attempts are made to relate reproductive traits to reproductive mode.

Genetics of Lupinus. V. Intraspecific Variability for Reproductive Traits in Lupinus nanus

DETERMINATIONS of the reproductive mode in higher hermaphrodite plants are usually based on observations of seed parent performance. While a seif-fertilising plant makes equal male and female gamete contributions to the next generation, so that male gamete contributions can be extrapolated from data on functional embryo sacs, this situation need not hold in outcrossing plants. Here the 1 1 ratio between male and female gametes obtains only at the level of the population and individuals or gentoypes in the breeding entity can function more largely as male than as female parents, or vice versa, at each other's expense. In partially self-fertilising plants this inequality can affect the outcrossing rate and in the present study such an effect is traced in the partially selfing California annual legume, Lupinus nanus Dougi. (ex. Benth.). The sporophytic outcrossing rate is usually measured in progeny tests by the frequency with which functional female gametes unite with male gametes produced by different sporophytes. For the purposes of the present study this rate is termed the female outcrossing rate. For the frequency with which functional male gametes unite with female gametes from a different sporophyte a separate estimator is derived and termed the male outcrossing rate. Environmental and genetic influences on rates of outcrossing in partial selfers have been demonstrated in numerous investigations (see e.g. Stevens and Finkner, 1953; Harding and Tucker, 1964; Vasek, 1967). In Lupinus nanus it seemed plausible that outcrossing would be affected by flower colour and pattern. Evidence on visual responses in honey bees (Daumer, 1956; Menzel, 1967) and bumble bees (Manning, 1956) suggested that the lupine mutant genotypes pink and spotless (Harding and Mankinen, 1967; Horovitz, 1969) were likely to attract pollinating insects less efliciently and, therefore, outcross at lower rates than a blue-flowered wild-type form. Our purpose was to verify such outcrossing differences and relate them separately to male and female outcrossing phases of mutant and wild-type forms. The results of the study are used to predict possible evolutionary outcomes of natural selection among genotypes that differ in the sex balance of their outcrossing phase.

/pdf/the-concept-of-male-outcrossing-in-hermaphrodite-higher-54qui8rm25.pdf

The concept of male outcrossing in hermaphrodite higher plants

Linearly nonrandom seed set patterns were found in pods of bee-pollinated Lupinus nanus Dougl. (ex Benth.) and Medicago sativa L. In a mixture of different genotypes of the California blue lupine and in a white-flowered mutant of tetraploid alfalfa, seed set was highest in the second ovule position from the peduncular end of the pod and declined toward the stylar end In M. sativa there was also a lateral non-randomness with seed set being much higher (P < 0 2%) in even-numbered ovule positions, which correspond to the upper valve of the pod if the fruit is viewed from the peduncular end, than in odd-numbered positions Highly significantly unequal seed set distributions between the two fruit valves also found in pods collected from the wild, belonging to M radiata L., M polymorpha L., and M. truncatula Gaertn. In left-handed (anticlockwise coiled) Medicago pods the higher seed set occurred in the upper valve, and in right-handed pods in the lower valve (fruits viewed from the peduncular end). In both L. na...

Effects of Ovule Positions in Fabaceous Flowers on Seed Set and Outcrossing Rates

Lupinus pilosus Murr varies in seed coat coloration, and white color was found to be transmitted as a monogenic recessive Using this trait as a genetic marker, we traced outcrossing in a wild population in Israel and in several experimental breeding entities derived from it The rate of outcrossing varied between 30% and over 60% in different environments In the absence of pollinators, the average seed set was two or three seeds per pod, whether or not hand tripping of flowers was practiced The estimates indicate that L pilosus is versatile in its mating system and can attain higher rates of outcrossing than those reported for other Old World lupines

Auguste Horovitz

Papers

Genetics of Lupinus. V. Intraspecific Variability for Reproductive Traits in Lupinus nanus

The concept of male outcrossing in hermaphrodite higher plants

Effects of Ovule Positions in Fabaceous Flowers on Seed Set and Outcrossing Rates

Genetics of Lupinus. XII. The Mating System of Lupinus pilosus