Cycle Life of Organisms Cell From Growth Modular Size Until Senescence

For readers interested in a brief synopsis of some themes to take away from plengdut post, the Following list may be useful. If somewhat more elaboration is desired, a review of the summaries at the end of each chapter is recommended.

• Every organism is unique in detail, but all share fundamental physiochemical and cytological properties and all have been shaped by evolution operating through differential reproductive success. The latter attribute provides a common basis for comparing analogous strategies among organisms.
• The term ‘individual’ has various connotations: numerical, genetic, physiological, and ecological. An informative way to view an individual is in the ecological context as the entity through its entire life cycle.
• Microorganisms have essentially all of the capability of macroorganisms in generating genetic variability, though adaptive evolution proceeds rather differently in the two groups. Variation specifically in the prokaryotes is transmitted in dynamic, unordered fashion, as opposed to the orchestrated manner, characterized by meiosis and gametogenesis, characteristic of the eukaryotic microorganisms and macroorganisms. In prokaryotes, the gene pool of distant relatives is tapped by horizontal transmission that introduces fundamentally new traits in a manner that apparently occurs rarely in eukaryotes. The fungi, as eukaryotic microorganisms, have a fairly fluid genome as exhibited by such processes or conditions as parasexuality, dikaryosis, and heterokaryosis.
• The occurrence of different ontogenetic programs among taxa means that in some cases somatic variation can be transmitted to the germline. This major implication, together with the different and in some cases multiple nuclear conditions during the life cycle, and the ubiquity of mobile DNA, mean that the concept of the genetic individual (genet), though remaining instrumental in terms of assessing how evolution acts, must be viewed as more fluid than as originally conceived.
• The quest for nutrients and energy is universal and the evolution of search strategies is a universal basis for comparisons. Optimal foraging theory is a cost/benefit analysis in energetic terms developed primarily as an economic optimization model to interpret the foraging behavior of certain animals. It can be construed broadly, merged with optimal digestion theory, and applied informally, conceptually, and empirically to all organisms. Broadly speaking, in terms of foraging, bacteria appear to do largely by metabolic versatility what animals accomplish by mobility and behavior, and plants, fungi, and other sessile organisms do by morphology.
• The major multicellular lineages evidently all arose from different unicellular progenitors. These transitions apparently occurred many times and in all three major domains of life, sometimes through a colonial intermediary and with occasional reversals to unicellularity. For about 2–3 billion years, life on Earth was microscopic and simple, preponderantly if not exclusively prokaryotic, with only basic forms of differentiation occurring, if at all. Conventional multicellularity was ultimately an evolutionary dead-end for prokaryotes; eukaryotes exploited growth form, in large part enabled by a fundamentally different cell structure as the building block, in a way that prokaryotes could not.
• Physical and chemical laws ultimately set the theoretical lower and upper size constraints on life: The low end, approached by some very small bacteria, is established by the minimum package size to contain cell machinery and associated molecular traffic; the upper limit on cell size is set mainly by the declining surface area-to-volume ratio as size increases.
• The small size of prokaryotes (and microorganisms in general) means that their world is dominated by molecular phenomena such as diffusion, surface tension, viscosity, and Brownian movement. Macroorganisms, in contrast, are conditioned primarily by gravity and scaling relationships. Extremely large size can be attained in some circumstances where gravitational and other constraints can be mitigated, such as by clonal growth on land or partial buoyancy in water.
• Allometric scaling denotes regular changes in certain proportions or traits as a function of size according to the general relationship of Y = aWb where exponent b is the scaling constant that defines the general nature of the relationship. A broadly based and biologically important example is the surface area law or the 2/3-power law (exponent b is 2/3) dictating the relative decline in surface area as volume increases (Principle of Similitude).
• Both the mean and upper size limits among biota as a whole have increased over geological time. The most important event spurring evolution of greater complexity arguably was the transition from unicellularity to multicellularity. This enabled organisms to increase in size, differentiate to segregate function by specialized cell type, and to develop increasingly sophisticated forms of intercellular communication and division of labor.
• Growth form, that is, the mode of construction and related morphology, sets fundamental opportunities and limits on the biology of organisms. All living things are basically either unitary or modular in design. The modular versus the unitary distinction is probably more biologically significant than a demarcation based on size (microorganisms versus macroorganisms). Bacteria and fungi are inherently modular in design and should be considered analogous with modular macroorganisms. 

The key evolutionary implications of modular design stem directly or indirectly from growth by iteration and sessility. The former include: (i) high phenotypic plasticity (shape; size; reproductive potential; growth as a population event); (ii) exposure of the same genet to different environments and selection pressures; (iii) iteration of germ plasm and the potentially important role for somatic mutation; and (iv) potential extreme longevity of the genet. The evolutionary implications of sessility include: (i) adaptations for the individual to reproduce without a mate or to reach a mate and disperse progeny by growth or transport (pollen; spores; seeds; bacteriophage movement of bacterial genes); (ii) potentially strong interactions with neighbors; (iii) inability to escape from adverse environments, hence the means to adapt in situ such as by dormancy; (iv) development of resource depletion zones and consequently resource search/exploitation strategies from relatively fixed positions.

• The inception of the life cycle is traceable to the simple cell cycle of prokaryotes. With the origin of multicellularity and eukaryote sex cell, life cycles became more expansive and identified with distinct phases related to developmental stage or nuclear condition. Apparently all organisms ultimately pass through a single-cell stage, a key event with profound genetic and developmental implications.
• When offspring are born into essentially the same habitat as the adults and do not undergo sudden ontogenetic change, the organism is said to have a simple life cycle (e.g., mammals). Where organisms have two or more ecologically distinct phases separated by an abrupt ontogenetic change, the life cycle is considered complex in ecological semantics (CLC). The rust fungi, many of whose parasitic life cycles consist of a sequence of five morphological states on two distinct hosts, illustrate the intricacies and principles of the complex life cycle. CLCs usually have been interpreted adaptively. However, the CLC may be an evolutionary dead-end and a prime example of how evolution can drive organisms to greater degrees of specialization.
• Senescence reflects deteriorative effects that decrease fecundity and the probability of survival with increasing age. At the population level, actuarial statistics showing an increasing mortality rate over time are indicative of senescence. If mortality is constant with age, the number of survivors declines exponentially and there is no basis for inferring from the data that senescence occurs.
• Vertebrates and many invertebrates such as the nematodes, crustaceans, insects, as well as some plants such as the determinate annuals, senesce. Evidence for senescence appears limited or nonexistent generally for most modular organisms, including benthic sessile invertebrates, most perennial plants, and microorganisms. For some organisms the probability of survival actually increases, along with fecundity, later in life. The terms ‘negligible senescence’ and ‘negative senescence’ have been used to describe such population dynamics. In an evolutionary context and as a first approximation, senescence is predicted to occur wherever the reproductive value of the individual diminishes with increasing age.
• The environment includes the physical and biological setting of an organism with which it is coupled and reciprocally interacting.
• How organisms experience and respond to environmental fluctuations are affected by their size and growth form. The macroorganism responds by virtue of a complex neural network; the counterpart of the neural network for the microorganism is the network of regulated biochemical pathways and metabolic controls. Modular organisms, composed of iterated parts and being sessile, respond differently to the environment than do unitary organisms.
• When individuals of a species experience relatively the same environment throughout their lifespan, locally adapted genotypes producing relatively invariant phenotypes known as ecotypes or races, are expected. Where individuals are exposed to an environment that varies principally spatially or temporally, phenotypic plasticity defined as any kind of environmentally induced phenotypic variation in behavior, physiology or morphology is expected. Plasticity dampens the effects of selection by uncoupling the phenotype from the genotype, providing for adaptation to variable environments. Such plasticity, frequently called ‘phenotypic heterogeneity’ in bacteriology, can occur within a genetically homogeneous cell population in a constant environment triggered by stochastic molecular noise.
• Dormancy, apparent as various manifestations of quiescence, is usually interpreted as a bet hedging strategy to adverse conditions. Particular phases of life cycles appear to match those environmental conditions for which they are suited but this is a result of natural selection acting on the individual’s ancestors over generations.

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Referance

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