摘要:

Variation in time (annual and seasonal) and space (between‐ and within‐population) is examined for the pollinator assemblage ofLavandula latifolia(Labiatae), an insect‐pollinated, summer‐ flowering, evergreen shrub of Mediterranean woodlands in southeastern Spain.Lavandula latifoliais pollinated there by nearly 85 species of comparatively long‐tongued Hymenoptera, Diptera and Lepidoptera.The diversity, composition and abundance of pollinators varied markedly between years. Lepidopteran taxa prevailed numerically in one year, hymenopterans in four years, and both groups had similar abundance in the remaining year. The vast majority of pollinators exhibited significant annual differences in average abundance. Only 35.7% of taxa were recorded in all of the six study years. The pollinator assemblage had marked seasonal dynamics. Important changes in species richness, abundance and composition took place over the flowering season ofL. latifolia(nearly 3 months). Only 21.7% of insect taxa occurred throughout the flowering season.Lavandula latifoliapopulations at different distances from water courses differed broadly in the abundance and composition of pollinators. Hymenopterans dominated numerically at the water‐distant sites, whereas lepidopterans prevailed in populations growing in the vicinity of streams. Only 40.7% of taxa were recorded at all of the four study populations. Within the same plant population, horizontal distances of the order of 25 m resulted in significant changes in pollinator composition.Variations at the four scales considered combine to produce, from the viewpoint ofL. latifolia, a spatio‐temporal mosaic of pollinators. As these differ broadly in frequency of pollen transfer, size of pollen loads deposited, and between‐flower flight distance patterns, spatio‐temporal variation will predictably result in inconsistent or contradictory selective pressures on the plant, thus hindering specialization in relation to parti

ISSN:0024-4066    

DOI:10.1111/j.1095-8312.1988.tb00461.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

A model is presented for the evolution and control of generative apomixis—a collective term for apomixis in animals and diplosporous apomixis in flowering plants. Its development takes into account data obtained from studies of apomictic‐like processes in sexual organisms and in non‐apomictic parthenogens, as well as data obtained from studies of generative apomicts. This approach provides insights into the evolution and control of generative apomixis that cannot be obtained from studies of generative apomicts alone.It is argued that the control of the avoidance of meiotic reduction during egg production in generative apomicts resides at a single locus, the identity of which can vary between lineages. This variation accounts for the observed variation between taxa in the pattern of avoidance of meiotic reduction. The affected locus contains a wild‐type allele that codes for meiotic reduction and excess copies of a mutant allele that codes for its avoidance. The dominance relationship between these is determined by their ratio and by the environment. Environmental differences between female generative cells and somatic cells are such that the phenotypic expression of the mutant allele is favoured in the former, while that of the wild‐type allele is favoured in the latter. This is important, for the locus is also involved in the control of mitosis which would be disrupted by the expression of the mutant allele in somatic cells. The requirement to maintain a viable pattern of growth and development explains why the wild‐type allele is retained by generative apomicts, and this in turn explains why the ability to produce meiotically reduced eggs is retained by facultative forms and why it appears to be suppressed in, rather than absent from, obligate forms. The requirement for excess copies of the mutant allele in generative cells explains why generative apomicts are typically polyploid, as this condition provides a simple and effective means of generating the correct balance of mutant and wild‐type alleles. Environmental effects can also lead to the dominance relationship between wild‐type and mutant alleles varying between generative cells. In plants, this can lead to the apomixis gene being expressed, and thus to meiotic reduction being avoided, in only some ovules. Meiotically reduced, as well as meiotically unreduced, eggs are produced when this occurs. If compatible and viable pollen is available the meiotically reduced eggs may be fertilized, resulting in these organisms reproducing as facultative apomicts.It is argued that the control and evolution of parthenogenesis in generative apomicts varies between taxa. In some, the parthenogenetic initiation of embryos may result from the acquisition of a parthenogenesis gene or genes; but there is no reason to believe that this is either a general or a common requirement. Indeed, in some it may be an ancestral trait, these apomicts differing from their sexual ancestors in the ability to mature, rather than in the ability to initiate, embryos from unfertilized eggs; or it may result from physiological or developmental changes induced, for example, by polyploidization, hybridization, or the avoidance of meiotic reduction. In some plants it may be induced by pollination (without fertilization) or by the activity of a developing endosperm.Although it is argued that most generatively apomictic lineages may have acquired this form of reproduction relatively easily, by the acquisition of a mutation at a single locus, it is argued that newly initiated lineages may often be reproductively inefficient. These will begin to accumulate mutations that improve the efficiency of apomictic reproduction. Thus several loci may be involved in the control of generative apomixis in established lineages, even though only a single locus was involved in its initiation in these lineages. Care must be taken to distinguish between these initiator and modifier genes when considering the evolution of generative apomixis. Finally, it is argued that although generatively apomictic lineages have easily acquired this form of reproduction, its evolution in some taxa may be so difficult, requiring the acquisition of mutations simultaneously at two or more loci, that these may never acquire it. Thus, evidence obtained from taxa that have successfully made the transition from sexual reproduction to generative apomixis that its evolution was straightforward should not be used as evidence that its evolution will always be relatively easily achieved. Its uneven taxonomic distribution indicates that it is much more easily evolved by some taxonomic groups

ISSN:0024-4066    

DOI:10.1111/j.1095-8312.1988.tb00462.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

The genusTaraxacumis a widely dispersed, ecologically variable taxon of some 2000 sexual and apomictic (agarnospermous) species. Data from numerous studies are used to examine the influences sexuality and apomixis have had on its evolution, geographical distribution and ecological diversification. A new explanation is given of the geographical distribution of sexual and apomictic forms, and the role of polyploidy in buffering apomicts against the effects of an accumulation of deleterious mutations is examined.

ISSN:0024-4066    

DOI:10.1111/j.1095-8312.1988.tb00463.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

The question of whether there are shape differences between populations ofLittorina saxatilisliving in different environments is examined by multivariate analyses of 13 morphological characters. Principal component analysis reveals that morphologic differences between populations from habitats with contrasting degrees of wave exposure are mainly due to a general size factor, including shell thickness. Utilizing the group structure among the snails, canonical variate analysis discloses that the main character excluding size that influences subpopulation differentiation is pointedness.

ISSN:0024-4066    

DOI:10.1111/j.1095-8312.1988.tb00464.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

The carnivorous pitcher plants of the Sarraceniaceae, Nepenthaceae and Cephalotaceae seem to have developed attraction and rewarding systems which resemble those of many flowers. Contrary to a previous view, Batcsian mimicry does not appear to apply to pitcher traps because: (a) there is no evidence of insects visiting traps by mistake, (b) no model is known for any one of the traps, (c) the duration of the trap activity and the structure of the plant community are not compatible with a deceptive mimetic status, (d) nectar which is a real reward is provided to visiting insects by the pitcher plants. It is suggested that these plants perform mutual rather than deceptive relations with the insect communities in their habitats. Insects benefit from nectar which is provided by pitchers and which serves for their nutrition in habitats where sources of floral nectar might be either uncommon or absent. At the same time the insects pay the plants in a small portion of their community which is ‘sacrificed’ as prey and consumed by the plants, which grow in nutrient deficient so

ISSN:0024-4066    

DOI:10.1111/j.1095-8312.1988.tb00465.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

We discuss an alternative source of randomness originating at the atomic level due to the diversity of stable isotopes. This randomness could contribute to mutation mechanisms through atomic scale variations of bond lengths and strengths, differences in the reaction rates, etc. In terms of purposeful experimentation, isotopic replacements in living organisms give an independent parametrization which allows correlation of global biological variables with the level of individual atoms. We review the work done so far, and suggest some further experimental possibilities by analogy with some isotopic effects and isotopic structuring in solid state physics.

ISSN:0024-4066    

DOI:10.1111/j.1095-8312.1988.tb00465a.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY