Tag Archives: developmental plasticity

Development – notes

“Development is phenotype change in a responsive (plastic) phenotype due to inputs from the environment and the genome. Individual metazoan development begins with a ready-made, highly organized and reactive phenotype provided by a mother in the form of an egg. Cross-generational continuity of the phenotype makes it ultimately impossible to estimate the relative importance of environmental and genetic influence on development, for the inherited phenotype is a product of genomic and environmental influences intertwined and unbroken since the origin of reproducing phenotypes.”

“Each developmental response of the phenotype represents a new branch, or switch point, in development, associated with the production of a modular trait. While switches produce discrete traits, switch mechanisms are phenotypically complex and polygenetically influenced, producing continuous variation in the dimension of traits and quantitative genetic influence on regulation of trait expression. The quantitative adjustability of liability for switch-controlled responses is one aspect of gene-environment interchangeability and change in plasticity during evolution, since environmental and genetic influences on a threshold (or the ability to exceed it) are inversely related.”

“Any comprehensive theory of adaptive evolution has to feature development. Development produces the phenotype variation that is screened by selection.”

“Genetic programming, the canalized epigenetic landscape, and the recies and blueprints contained in the genes–all are metaphors for development. Development is the missing link between genotype and phenotype”

“a concept of development that connects it to mechanisms, on one hand, and natural selection and evolution on the other.”

“a strong emphasis on the genome means that environmental influence is systematically ignored”

“The problem of evolutionary developmental biology has been not too much genetics but too little of the other factors of development, the organized flexible phenotype that is the setting for gene action, the influence of higher levels of organization on how genes are expressed, and the external environment as a source of developmental building blocks and information as crutial as genetic mutation for the origins of evolutionary novelties.”

“individual development always starts with an organized phenotype inherited from previous generations. That phenotype is highly responsive to new inputs, which can be either genomic or environmental… These inputs cause responses that correspond to new switch or decision points in development and give rise to new phenotype traits. The result is development of a phenotype that is a mosaic of switch-determined modular traits.”

Continuity of the Germ Line and Neo-Weismannian Reduction

“The idea of continuity of the genetic material is an indispensible part of any theory of biological organization.”

“Neo-Weismannian reduction – germ cells immortal, gene material survives while the phenotype comes and goes. A phenotype-centered view emphasizes connectedness between life cycles where there is a phenotypic continuity among generations.”

Continuity of the Phenotype

“Individual development always begins with an inherited bridging phenotype — a responsive, organized cell provided by the parent in the form of an egg, a newly divided cell, or a set of cells that springs entirely from the previous generation … is the active and organized field upo  which the zygote/offspring genome products and subsequent environments eventually act.”

“organisms are lineage products, not discrete entities at all”

“the products of plant meiosis in both sexes form multicellular structures”

“Gymnosperms are more like sexually reproducing animals in that the supplies to the embryo are maternal in orgin”

“Paternal effects my be more widespread than heretofore appreciated”

“Presumably factors transmitted in the male’s seminal fluid modify the female’s behavior and female offspring are somehow affected … Cross-generational hormone effects”

“parental contributions to the inherited phenotype obviously present opportunities for manipulations, and maternal manipulation of offspring via substances introduced into the egg may be a common phenomenon”

“the life cycle of an individual does not begin with fertilization and end with the adult … it is just one loop in a continuous string of ontogenies linked by these gametic phenotypic bridges between generations”

“Development is a process where every change builds upon a previous organization. Sometimes the offspring phenotype is manipulated by products of genes , and sometimes by factors in the environment. These factors nudge and join a pre-existing structure with some elements so responsive and internally active that they are sometimes said to organize themselves.”

“all order proceeds from pre-existing order”

Britten (1998) “All living systems except a few parasites (viruses and DNA elements) have an unbroken lineage from cell to cell going back to some cell precursor, perhaps part of the origin of life, many billions of years ago. So the starting point can only be a complete cell. Thus in oogenesis the first units are bound to pre-existing structure and the pre-existing parental cell informs the early stages”

“As soon as proteins were synthesized by replicating molecules–the birth of the somatic phenotype–it was possible to endow replicates with substances that could be advantageously carried over and used in a subsequent generation”

“Phenotypes, including those cross-generational bridging phenotypes, are of course characterized by plasticity–the ability to react to stimuli with a change in form, state, movement, or rate of activity. This property of the bridging phenotype makes it susceptible to the new inputs that initiate ontogenetic change. Some inorganic structures also react to inputs from outside themselves, and such materials undoubtedly gave rise to earliest life.”

“The same kind of built-in sensitivity that make phenotypes responsive to the environment can make them responsive to inputs specified by genes, to manipulation by parents and parasites, and to internal interactions among parts … Responsiveness to all of these different influences give rise to development and to variation that fuels selection and evolution.”

“the zygotic genome is constrained to play upon the responsive structure that is in place when particular genes are expressed”

“Gene products are made from elements imported from the environment. Then, once a gene product is assembled, it’s biological significance depends on the reactivity to it of the phenotype.”

“Without a responsive phenotype with a particular configuration, and specific environmental supplies, the selection-hewn codex written in the genes is meaningless”

The Bridging Phenotype in Early Ontogeny

“active survival in a new environment”

“mating interactions between eggs and the spermazoa”

“mate selection behavior, polyspermy”

“sometimes several cell divisions before genes become active”

“Although a complex motile sperm cell looks like an independent little individual, the genes of the animal spermatozoan in most species and physically condensed and completely inert during the gamete stage”

“the morphological, biochemical, and behavioral phenotype of the spermatozoan is a product of the parental phenotype, not the genes within the sperm. The parental phenotype is, in turn, a product of both its environment and its genes”

“the egg phenotype qualifies as a full-fledged, organized and active individual. It may be morphologically equipped for dispersal, respirtation, active transport of water and nutrients and interactions with sperm. The egg micropyle, or entryway for the sperm, is sometimes an elaborate and species-specific structure.”

“Contrary to the image of the passive egg being penetrated by an active sperm, the animal egg can take an active and selective role in fertilization, engulfing (or not engulfing) a sperm and engaging in other activities that are necessary for fertilization to occur”

“egg choosing its mate (choice being differential responsiveness to different alternatives)”

“[in flowering plants] fertilization is potentially influenced by the maternal phenotype, though not directly by the egg phenotype itself, for the mother supplies substantial resources for the pollen-tube growth that enables pollen to reach eggs”

“blurring of the line between phenotypes scores the importance of continuity of the phenotype”

“In collared flycatchers, Ficedula albicollis, for example, adding one egg to the mother’s clutch affects the clutch size of her adult daughters, reducing it by an average of one fourth of an egg”

“In migratory locusts the maternally transmitted effects of crowding on the adult phenotype accumulate across generations, with four generations of crowding required to produce the full expression of the migratory phenotype”

“Such phenomena provide at least a glimpse of the indefinitely extended consequences of continuity of the phenotype, in which events are ‘both the effect of earlier, and the cause of later developmental transactions’”

“The uniqueness of the germ line compared to the phenotype line as a factor in evolution lies not in its continuity, a property shared with the phenotype, but in its relative immutability during development, the faithfulness of its replication, and the dependence of its rate of multiplication on the sucess of the phenotype as a whole. As a result of these three qualities, the population of genotypes reflects their history of selective screening across time. But the continuity of the germ line depends on the continuity of the phenotype that bears it.”

“A corollary of continuity of the phenotype is that phenotypic structures are the units of reproduction.”

“Genes replicate, but they cannot reproduce themsleves across generations. Even in gametes they are packaged in phenotypes as eggs, spores, pollen, or sperm and sometimes vegatative fragments built by the parent. Over the long run, the survivial and frequency of genes in a population depends on the packaging–the properties of phenotypes that reproduce.”

“In meiotic drive a selfish genes behavior may bias its probability of entering a gamete and being differentially transmitted to the next generation, but its transmission nonetheless depends on the success of the reproducing phenotype and its cross-generational continuity”

“screening off – genes are screened off from being the primary agents of cross-generational reproduction bt the dependency of their reproduction on that of phenotypes. Because of screening off, reproducing phenotypes are appropriately considered the units of selection, even though evolution, a response to selection, is defined in terms of change in the frequency of genes.”

The Dual Nature of Regulation

“How do environmental supplies, ordered by the genome, affect the highly reactive phenotype that exists before they arrive?”

“Development is change in the phenotype as it is influenced by genomic and environmental inputs over time. The effects of these inputs depend crucially on the organization of the preexisting phenotype.”

“The concept of information is meaningfull only with respto a receiver phenotype that is organized to respond in a particular way”

“The preexisting phenotype is a transducer of both environmental and genomic information. The very specificity of gene action, and of environmental effects as well, depends as much on that structure as on the input that causes a response.”

“Tissue-specific gene expression is a product of local phenotypic divergence in the properties of cells.”

“Neither genes nor environmental conditions have any developmental significance without a phenotype already organized to respond”

“The nature-nurture dichotomy disappears with the realization that the developing phenotype responds to both internal and external stimuli in much the same way. As a result, genomic and environmental factors are interchangeable during evolution”

“Despite the confusion engendered by the nature-nurture dichotomy, there is good reason to maintain the distinction between external environment and the genome, at least in evolutionary discussions”

“Only the genetic inputs have been screened by selection. Selection may favor phenotypes that actively incorporate certain environmental elements, but can only do this via gene frequency change.”

“environmental effects on development can be as specific and essential as genetic effects.”

“The dual nature of regulation is evident from the nature of phenotype determination at switch points. Phenotypic or trait determination means the adoption of one of two alternative states or pathways at a decision point. Determination can be distinguished from influence on regulation. A single genetic locus may determine whether or not a discrete trait is expressed if the alternative alleles at that locus inevitably cause a threshold to be passed or not passed, and that trait developed or not developed. The switch may nonetheless be influenced by other genetic and environmental factors.”

“Possession of a particular trait rather than an alternative trait can be either genetically or environmentally determined, but regulation–the mechanism or the process–can never be determined by genes or environment alone, because the mechanism is an aspect of structure, and structure is always a product of both genetic and environmental influence”

“In development, genes and environment have complementary quantitative effects on switches, such that an increase in the influence of one implies a commensurate decrease in the influence of the other.”

“environmental and genetic factors in a sense compete with each other for control of regulation”

“it follows from the fact that influence on trait determination can come from two and only two sources–genome and environment–and threshold effects are all-or-none in nature”

“sex ratio in map turtles – in most conditions, heritability of sex ratio is zero but within  small narrow range of temperatures it is quite large” in the wild there is selection by the female for either sunny or shaded nests and this narrow temperature range rarely happens. no selection at temperature extremes but selection around the switch points.

“Such careful controls have to be imposed to isolate genetic effects from the environmental ones that the difficulty of doing so  should be considered a major datum in itself”

“most if not all environmentally controlled alternative phenotypes yeild less than 100% of the expected morph under  given condition, suggesting genetic variation for morph determination”

“conversely, genetic traits invariably have demonstrable susceptibility to environmental influence if sought”

“norm of reaction of a phenotype — the array of phenotypes that will be developed by the genotypes over an array of environments”

“plasticity, or the set of responses provoked by environmental variation, is determined by the genotype. if this is true, different genotypes may have different degrees and forms of plasticity; then, it follows that plasticity itself is subject to selectin and evolutionary change”

reaction norms associated with different genotypes – “different genotypes have different degrees of plasticity” – “the pattern of plasticity varied with some genotypes” – “This clearly shows the deterministic role of genotype in plasticity” and “the deterministic role of the environment in setting not only the form of the phenotype but also the range of gentic variation expressed and, hence, exposed to selection.”

“When there is overlap in reaction norms, the different genotypes produce overlapping distributions of phenotypes. In environmental conditions where the lines of genotypes intersect, the genotype differences would be ‘invisible’ to selection”

“When dealing with plastic traits, then, one cannot ignore the dual role of the environment in determining the strength of selection and the course of evolution: the environment is not only the agent of selection in the sense of being the arena where phenotypes are evuated in a game of survival and reproductive success. It is also an agent of development, which by interacting differently with different available genotypes sets the phenotypes in the positions where they can be seen by selection”

“There is no known method to estimate the proportional total influence of environment versus genes on individual or trait development”

“Heritability could be zero, for example, in an extreme environment or one that fluctuates so that negative and positive effects on heritability exactly cancel.”