Fact sheet 5: Life history strategies

Morgane van Antro

In ecology, the life history of a species refers to its demography features and include traits such as time of first reproduction, number of offspring per reproduction and in total, generation time and lifespan1.  These life histories are shaped by natural selection and reflects how a member of a species needs to distribute their resources among growth, survival and reproduction. In a natural environment, however, the resources needed are limited in supply and thus a trade-off on how the resources are distributed between growth, body maintenance and reproduction is needed in order to maximise fitness. How a species distributes its resources is known a life-history strategy and is the collection of life-history traits which are adapted to the environment a species finds itself in [1]. An important trade-off made by many animals and plants is between growth and reproduction. In most organisms, an increase in growth is countered by a change in the present reproduction capabilities, and vice-versa [2]. In plants, germination is the sprouting of a seed and is the earliest growth phase of a plant. Often germination occurs after a phase known as seed dormancy, which allows seeds to remain inactive until environmental conditions are optimal for growth to start. The transition from dormancy to germinations depends on both genetic and epigenetic factors. For example, in Arabidopsis thaliana the deacetylation of histones leads to the silencing of genes that controls hormones such as ethylene, abscisic acid and gibberellin, which are important to maintain dormancy [3,4]. The next pivotal step in angiosperm development is flowering. Understanding the evolution of time-to-reproduction is a key area in life-history research, and any direct or indirect selection pressures could induce changes in reproduction times during an individual’s lifetime [4,5]. In plants, this is measured via plant flowering time and vary widely among taxa. Some plants are annuals that flower only once and complete their life-cycle after reproduction in that year. Others are perennial that live for many years and flower repeatedly. Understanding why and how such variation in flowering behaviour emerged is a central interest in both ecology and evolution. Arabidopsis thaliana, an annual herb, provides the opportunity to determine the (epi)genetic basis that contributed to natural variations in the timing of flowering3,6. Comparing these finding to a perennial plant such as Arabis aplina, a close relative of A. thaliana, allows to determine which genetic and epigenetic marks (regulation of the FRIGIDA  FLOWERING LOCUS C genes as well as rate at which histones turn from an activated to a repressed state after vernalisation) are key factors in explaining natural variation of flowering times both in annual and perennial plants [6]. While all angiosperms flower, not all of them depend on sexual reproduction to propagate. Indeed, another important life history trait is the mode of reproduction. Plant can reproduce sexually and/or asexually. Depending on the mode of reproduction, the importance of epigenetic mechanisms will vary. This is particularly true when it comes to epigenetic reprogramming which occur during the formation of the germline [7]. This reprogramming is important for transmitting epigenetic information between cells/generations but more importantly, is essential for resetting epigenetic marks in order to reduce the risk of maintaining and transmitting dangerous epigenetic alleles [7,8]. It also, however, reduces the chances of transmitting novel and adaptive epigenetic alleles to the next generation. In general, epigenetic modifications of the genome are generally stable in somatic cells. In germs cells and early embryos, however, epigenetic reprogramming occurs on a genome-wide scale, and includes demethylation and remethylation of DNA and remodeling of histones [7]. In contrast to mammals, plants epigenetic reprogramming/resetting is classified as incomplete as the germline originates from adult somatic cells [8]. This means that the cells have gone through meiosis and thus environmental imprinting of the cells that will develop into a new individual is possible. Differences in the intensity of the epigenetic reprogramming can also be expected between plants with different reproductive modes [9]. Indeed, for epigenetic reprogramming to occur, a germline and thus sexual reproduction must occur. Yet some plants reproduce asexually (clonal or apomixes) where germline formation is incomplete or inexistent and thus epigenetic reprogramming, or at least resetting, could be bypassed [9,10]. This could in turn allow for a higher chance of transgenerational epigenetic inheritance to occur.  Inherited epigenetic marks have the potential to allow for populations to adapt to a changing environment. This might be particularly true for plants that reproduce asexually (including invasive plants) and to a lesser extend in apomicts. Theoretically, asexual plants are evolutionary dead end due their lower evolutionary capabilities compared to sexual species [11,12]. Perhaps, stably inherited epigenetic marks would allow for asexually reproducing plants to outweigh these evolutionary disadvantages. All in all, more and more studies are demonstrating the role of epigenetics when it comes to specific life-history strategies. These findings could change our way of seeing how these strategies have evolved and how they are controlled.

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