Abstracts from a few publications
[6]
Johnston, M.O., and D.J. Schoen. 1995. Mutation rates and dominance levels of genes affecting total fitness in two angiosperm species. Science 267: 226-229. Abstract. Theories about the evolution of sex and the effects of inbreeding depend on knowledge of the mutation rate and dominance level of deleterious alleles affecting total fitness. In two species of largely self-fertilizing annual plants, minimal estimates of such mutation rates were found to be 0.24 to 0.87 per sporophyte genome per generation, but confidence intervals exceeded 1.0 in each of the four populations. Dominance levels were near zero in one species and intermediate (0.28 to 0.35) in the other. These results suggest that the detrimental effects of inbreeding are a result of new partially recessive mutations rather than overdominance. [Pubs List] [Home]
[10]
Johnston, M.O., B. Das, and W.R. Hoeh. 1998. Negative correlation between male allocation and self-fertilization in a hermaphroditic animal. Proc. Natl. Acad. Sci. USA 95: 617-620. Abstract. Sex-allocation theory predicts that the evolution of increased rates of self-fertilizartion should be accompanied by decreased allocation to male reproduction (sperm production and broadcast). This prediction has found support in plants but has not previously been tested in animals, which, in contrast ot biotically pollinated plants, are free of complications associated with incorporating the costs of attractive structures such as petals. Here we report rates of self-fertilization as well as proportional allocation to male reproductive tissues within populations of the simulataneous hermaphrodite Utterbackia imbecillis, a freshwater mussel. Individuals from populations with higher selfing rates devoted a lower proportion of reproductive tissue to sperm production (correlation = --0.99), in support of theory. [Pubs List] [Home]
[12]
Johnston, M.O. 1998. Evolution of intermediate selfing rates in plants: pollination ecology versus deleterious mutations. Genetica 102/103: 267-278. Abstract. The evolutionarily stable rate of self-fertilzation is studied in phenotypic models that incorporate pollination ecology as well as the correlated evolution of inbreeding depression and the population mean selfing rate. Inbreeding depression is assumed to be caused by continual mutation to deleterious, partially recessive alleles. Several mutation rates and dominance levels are included. Two separate ecological cases are studied: how selfing rate affects proportion of ovules fertilized (pollination assurance, seed discounting) and how selfing rate affects male outcrossing success through pollen discounting. Evolutionarily stable rates are invariably zero or intermediate in two circumstances, namely when increased selfing causes (1) a decrease in the proportion of ovules fertilized or (2) an increase in pollen discounting and, therefore, a disproportionate decrease in male outcrossing success. Complete selfing is stable when selfing increases the proportion of ovules fertilized for all selfing rates. Stable selfing is zero or one in cases where the selfing rate has no effect on the proportion of ovules fertilized or when pollen discounting does not increase with selfing. Higher inbreeding depression tends to decrease the optimal selfing rate, and lower inbreeding depression (higher dominance coefficients and lower mutation rates) is more favorable to the existence of stable intermediate selfing rates. Approaches such as this that explicitly incorporate the interdependence of selfing, ovule fertilization, and male outcrossing may help explain the persistence of intermediate selfing rates in animal-pollinated plants. Key words: dominance - inbreeding depression - mutation rate - pollen discounting - pollination - self-fertilization [Pubs List] [Home]
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Li, P., and M.O. Johnston. 1999. Evolution of meiosis timing during floral development. Proceedings of the Royal Society B: Biological Sciences 266: 185-190. Abstract. Meiosis divides the haploid and diploid portions of the life cycle in all sexual organisms. In angiosperms meiosis occurs during flower development, the duration of which varies widely among species and is affected by environmental conditions within species. For 36 species representing 13 angiosperm families, we determined the time at which meiosis ceased in the anthers as a fraction of the total time from floral primordium initiation (beginning of development) to flower opening (end). It was found that this fraction, rather than being continuously distributed among species, occurred in three discrete classes despite wide variations within and among species in absolute developmental durations. Each species was characterized by a single timing class. For all species within a given timing class, therefore, the durations before and after the end of microsporocyte meiosis existed in constant ratio. Each timing class was found in phylogenetically distant species, and, conversely, a plant family often contained more than one class. Timing class was not related to ploidy level, inflorescence architecture, pollination syndrome or mating system. These findings show that either the durations before and after microsporocyte meiosis are regulated by the same exogenous process, or one duration determines the other. They further imply that the underlying developmental processes have evolved in a limited number of ways among flowering plants.
[22]
Whittle & Johnston. 2002. Male-Driven Evolution of Mitochondrial and Chloroplastidial DNA Sequences in Plants. Mol. Biol. Evol. 19 (6): 938-949. Abstract. Although there is substantial evidence that in animals male-inherited neutral DNA evolves at a faster rate than female-inherited DNA, the relative evolutionary rate of male- versus female-inherited DNA has not been investigated in plants. We compared the substitution rates at neutral sites of maternally and paternally inherited organellar DNA in gymnosperms. The analysis provided substantial support for the presence of a higher evolutionary rate in both the mitochondrial and chloroplastidial DNA when the organelle was inherited paternally than when inherited maternally. These results suggest that, compared to eggs, sperm tend to carry a relatively greater number of mutations in mitochondrial and chloroplastidial DNA. The existence of a male mutation bias in plants is remarkable because, unlike animals, the germ-lines are not separated from the somatic cells throughout an individual’s lifetime. The data therefore suggest that even a brief period of male and female germ-line separation can cause gender-specific mutation rates. These results are the first to show that, at least in some species, germ-lines influence the number of mutations carried in the gametes. Possible causes of male-mutation bias in plants are discussed. [Pubs List] [Home]
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Simons & Johnston. 2003. Suboptimal timing of reproduction in Lobelia inflata may be a conservative bet-hedging strategy. Journal of Evolutionary Biology 16: 233-243. Abstract. Age and size at reproduction are important components of fitness, and are variable both within and among angiosperm species. The fitness consequences of such life-history variation are most readily studied in organisms that reproduce only once in their lifetime. The timing of the onset of reproduction (bolting) in the monocarpic perennial, Lobelia inflata, occurs over a range of dates within a season, and may be postponed to a later season. Empirical relationships among life-history traits, derived from over 950 wild-growing and experimentally manipulated plants in the field, are used to model an optimal changing size threshold (norm of reaction) for bolting over the growing season. Comparisons are made between observed and expected norms of reaction governing bolting. An apparently suboptimal bolting schedule that precludes bolting beyond an early (conservative) date is observed, and is found to be qualitatively consistent with conservative bet hedging under unpredictable season lengths. On this basis we propose the schedule of bolting as a plausible example of a conservative bet-hedging strategy. The results underscore the critical need for long-term studies of fluctuating selection to distinguish suboptimality from bet hedging. [Pubs List] [Home]
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Whittle & Johnston. 2003. Broad-scale analysis contradicts the theory that generation time affects molecular evolutionary rate in plants. J. Mol. Evol. 56:223-233. Abstract. Several studies of plant taxa have concluded that generation time, including annual/perennial life history, may explain molecular evolutionary rate variation in selectively neutral DNA. Unlike in animals, there is little theoretical basis for why generation-time effects would exist in plants. Furthermore, previous reports fail to establish the generality of a generation-time effect in plants because of the small size of the datasets, a large proportion of which compared very widely divergent taxa differing in many characteristics other than generation time. Using 24 phylogenetically independent species pairs, each containing a species with an annual and a species with a perennial life history, and nine species pairs, each containing a tree species with a short and a long minimum generation time, we found no evidence that generation time is related to molecular evolutionary rate variation of the nuclear 18S ITS1 and ITS2 regions. This analysis strongly contradicts the growing belief that evolutionary rates are affected by generation time in plants. Possible reasons for the absence of generation-time effects are discussed that includes an evaluation of the cell-division theory. [Pubs List] [Home]
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Whittle & Johnston. 2003. Male-biased transmission of deleterious mutations to the progeny in Arabidopsis thaliana. Proc. Natl. Acad. Sci. USA 100: 4055-4059. Abstract. The extent and cause of male-biased mutation rates, the higher number of mutations in sperm than in eggs, in animals is currently an active and controversial subject. Recent evidence indicates that this male (sperm) bias not only occurs in animals but also occurs in plants. The higher mutation rate in plant sperm was inferred from rates of evolution of neutral DNA regions, and the results were confined to the mitochondria and chloroplasts of gymnosperms. The relative transmission rates of deleterious mutations, however, which have substantial evolutionary consequences, have rarely been studied. Here, an investigation is described using the hermaphroditic self-compatible flowering plant Arabidopsis thaliana, in which we artificially increased the rate of mutation in pollen (i.e., sperm donor) and maternal (i.e., egg donor) parents, using two kinds of ultraviolet (UV) irradiation in parallel and separate experiments, and assessed the deleterious effects on fitness of the F2 generation. The results strongly indicate that more deleterious induced mutations are transmitted to the progeny by a sperm than by an egg. These findings provide the first experimental evidence that more deleterious mutations are inherited from sperm than from an egg in any organism. Possible causes underlying this male bias are discussed. [Pubs List] [Home]
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Ashman, T.-L., T. M. Knight, J.A. Steets, P. Amarasekare, M. Burd, D. R. Campbell, M. R. Dudash, M. O. Johnston, S. J. Mazer, R. J. Mitchell, M. T. Morgan, and W. G. Wilson. 2004. Pollen limitation of plant reproduction: ecological and evolutionary causes and consequences. Ecology 85: 2408-2421. Abstract. Determining whether seed production is pollen limited has been an area of intensive empirical study over the last two decades. Yet current evidence does not allow satisfactory assessment of the causes or consequences of pollen limitation. Here, we critically evaluate existing theory and issues concerning pollen limitation. Our main conclusion is that a change in approach is needed to determine whether pollen limitation reflects random fluctuations around a pollen–resource equilibrium, an adaptation to stochastic pollination environments, or a chronic syndrome caused by an environmental perturbation. We formalize and extend D. Haig and M. Westoby’s conceptual model, and illustrate its use in guiding research on the evolutionary consequences of pollen limitation, i.e., whether plants evolve or have evolved to ameliorate pollen limitation. This synthesis also reveals that we are only beginning to understand when and how pollen limitation at the plant level translates into effects on plant population dynamics. We highlight the need for both theoretical and empirical approaches to gain a deeper understanding of the importance of life-history characters, Allee effects, and environmental perturbations in population declines mediated by pollen limitation. Lastly, our synthesis identifies a critical need for research on potential effects of pollen limitation at the community and ecosystem levels. [Pubs List] [Home]
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Knight, T. M., J. A. Steets, J. C. Vamosi, S. J. Mazer, M. Burd, D. R. Campbell, M. R. Dudash, M. O. Johnston, R. J. Mitchell and T.-L. Ashman. 2005. Pollen limitation of plant reproduction: pattern and process. Annual Review of Ecology, Evolution and Systematics 36: 467-497. Abstract. Quantifying the extent to which seed production is limited by the availability of pollen has been an area of intensive empirical study over the last few decades. While a prominent theoretical argument predicts that pollen augmentation should not increase seed production, a large number of empirical studies report significant and strong pollen limitation. Here, we use a variety of approaches to examine the correlates of pollen limitation in an effort to understand its occurrence and importance in plant evolutionary ecology. In particular, we examine the role of recent ecological perturbations in influencing pollen limitation, the complications in interpreting experimental tests of pollen limitation, and the relation between pollen limitation and plant traits. We find that the magnitude of pollen limitation observed in natural populations depends on both intrinsic and extrinsic factors. [Pubs List] [Home]
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Johnston, M.O., E. Porcher, P-O. Cheptou, C.G. Eckert, E. Elle, M.A. Geber, S. Kalisz, J.K. Kelly, D.A. Moeller, M. Vallejo-Marín and A.A. Winn. 2009. Correlations among fertility components can maintain mixed mating in plants. American Naturalist 173:1-11. Abstract. Classical models studying the evolution of self-fertilization in plants conclude that only complete selfing and complete outcrossing are evolutionarily stable. In contrast to this prediction, 42% of seed-plant species are reported to have rates of self-fertilization between 0.2 and 0.8. We propose that many previous models fail to predict intermediate selfing rates because they do not allow for functional relationships among three components of reproductive fitness: self-fertilized ovules, outcrossed ovules and ovules sired by successful pollen export. Because the optimal design for fertility components may differ, conflicts among the alternative pathways to fitness are possible, and the greatest fertility may be achieved with some self-fertilization. Here we develop and analyze a model to predict optimal selfing rates that includes a range of possible relationships among the three components of reproductive fitness as well as the effects of evolving inbreeding depression caused by deleterious mutations, and selection on total seed number. We demonstrate that intermediate selfing is optimal for a wide variety of relationships among fitness components, and that inbreeding depression is not a good predictor of selfing-rate evolution. Functional relationships subsume the myriad effects of individual plant traits and thus offer a more general and simpler perspective on mating-system evolution. [Pubs List] [Home]
Bartkowska, M.P., and M.O. Johnston. 2009. Quantitative genetic variation in populations of Amsinckia spectabilis that differ in rate of self-fertilization. Evolution 63: 1103-1117. Abstract. Self-fertilization is expected to reduce genetic diversity within populations and consequently to limit adaptability to changing environments. Little is known, however, about the way the evolution of self-fertilization changes the amount or pattern of the components of genetic variation in natural populations. In this study, a reciprocal North Carolina II design and maximum-likelihood methods were implemented to investigate the genetic basis of variation for 15 floral and vegetative traits in four populations of the annual plant Amsinckia spectabilis (Boraginaceae) differing in mating system. Six variance components were estimated according to Cockerham and Weir’s “bio” model c. Compared to the three mixed-mating populations, we found significantly lower levels of nuclear variance for several traits in the nearly completely self-fertilizing population. Furthermore, for 11 of 15 traits we did not detect nuclear variation to be significantly greater than zero. We further found high maternal variance in one of the mixed-mating populations for several traits, and little dominance variance in any population. These results are in agreement with the evolutionary dead-end hypothesis for highly self-fertilizing taxa. [Pubs List] [Home]