Adaptive evolution of larvae and life cycles

The larval patterns of marine invertebrates pose intriguing questions for both evolutionary and developmental biologists. However, combined investigations have been rare. Quantitative models analyze the selective factors that drive evolutionary change in larval nutrition and timing of metamorphosis. Developmental studies describe the morphogenesis characterizing ancestral and derived larval patterns. Rigorous evolutionary analysis of the transition to derived modes of development is lacking and detailed developmental and ecological data are needed to test and refine theoretical models. A major challenge facing studies of life cycle evolution is the elucidation of the genetic structure and covariance of important developmental and larval traits.     

How gene families evolve

Theories and facts of gene family evolution are reviewed. Concerted evolution is commonly observed for gene families which originated a long time ago, however there are many different types of multigene families, from uniform to diverse. The rate of homogenization by unequal crossing-over, gene conversion, etc. has been evolutionarily adjusted for each gene family. When new functions are needed by organisms, gene families may evolve into superfamilies, in which no further concerted evolution takes place, and each member of the family may acquire an indispensable function. The homeobox-containing gene family is a most exciting example of such superfamily.     

How many genes evolve?

Degree of genetic similarity was estimated by electrophoretic comparison of 8–22 homologous proteins among several species of bacteria, slime molds, bony fishes, and bats. Genetic proximity generally varied directly with phylogenetic proximity, but even between closely related species more than half of the genetic loci were different. Although the times of divergence of the species studied are not known, it seems unlikely that this amount of genetic change could be effected entirely by selection. It also appears that many of the enzyme differences have little or no effect on activity. The results thus tend to support the position that a large part of evolutionary change is effected by random incorporation of selectively neutral mutations.Supported in part by NIH Grant GM 15597.     

How could the Gompertz-Makeham law evolve

In line with the origin of life from the chemical world, biological mortality kinetics is suggested to originate from chemical decomposition kinetics described by the Arrhenius equation k=A*exp(−E/RT). Another chemical legacy of living bodies is that, by using the appropriate properties of their constituent molecules, they incorporate all their potencies, including adverse ones. In early evolution, acquiring an ability to use new molecules to increase disintegration barrier E might be associated with new adverse interactions, yielding products that might accumulate in organisms and compromise their viability. Thus, the main variable of the Arrhenius equation changed from T in chemistry to E in biology; mortality turned to rise exponentially as E declined with increasing age; and survivorship patterns turned to feature slow initial and fast late descent making the bulk of each finite cohort to expire within a short final period of its lifespan. Numerical modelling shows that such acquisition of new functions associated with faster functional decline may increase the efficiency of investing resources into progeny, in line with the antagonistic pleiotropy theory of ageing. Any evolved time trajectories of functional changes were translated into changes in mortality through exponent according to the generalised Gompertz-Makeham law μ=C(t)+Λ*exp[−E(t)], which is reduced to the conventional form when E(t)=E₀−γt and C is constant. The proposed model explains the origin of the linear mid-age functional decline followed by its deceleration at later ages and the positive correlation between the initial vitality and the rate of ageing.     

How did parasitic worms evolve?

Nematodes are important parasites of humans and other animals. Nematode parasitism is thought to have evolved by free‐living, facultatively developing, arrested larvae becoming associated with animals, ultimately becoming parasites. The formation of free‐living arrested larvae of the nematode Caenorhabditis elegans is controlled by the environment, and involves dafachronic acid (DA) and transforming growth factor (TGF)‐β signalling. Recent data have shown that DA acid signalling plays a conserved role in controlling larval development in both free‐living and parasitic species. In contrast, TGF‐β signalling does not seem to be conserved; this difference perhaps points to how nematode parasitism did evolve.     

From antagonistic larvae to mutualistic adults: coevolution of diet niches within life cycles

Population structures largely affect higher levels of organization (community structure, ecosystem functioning), especially when involving ontogenetic changes in habitat or diet. Along life cycles, partners and interaction type may change: for instance Lepidopterans are herbivores as larvae and pollinators as adults. To understand variations in diet niche from larvae to adults, we model a community of two plant species and one stage‐structured insect species consuming plants as juvenile and pollinating them as adult. We model the coevolution of juvenile and adult diet specialization using adaptive dynamics to investigate when one should expect niche partitioning or niche overlap among life stages. We consider ecological and evolutionary implications for the coexistence of species. As predicted based on indirect effects among stages, we find that juvenile diet evolution increases niche overlap and favours the coexistence of plants, while the evolution of the adult diet decreases niche overlap and reduces plant coexistence, because of positive feedbacks emerging from the mutualistic interaction.     

Aggregative cycles evolve as a solution to conflicts in social investment

Multicellular organization is particularly vulnerable to conflicts between different cell types when the body forms from initially isolated cells, as in aggregative multicellular microbes. Like other functions of the multicellular phase, coordinated collective movement can be undermined by conflicts between cells that spend energy in fuelling motion and ‘cheaters’ that get carried along. The evolutionary stability of collective behaviours against such conflicts is typically addressed in populations that undergo extrinsically imposed phases of aggregation and dispersal. Here, via a shift in perspective, we propose that aggregative multicellular cycles may have emerged as a way to temporally compartmentalize social conflicts. Through an eco-evolutionary mathematical model that accounts for individual and collective strategies of resource acquisition, we address regimes where different motility types coexist. Particularly interesting is the oscillatory regime that, similarly to life cycles of aggregative multicellular organisms, alternates on the timescale of several cell generations phases of prevalent solitary living and starvation-triggered aggregation. Crucially, such self-organized oscillations emerge as a result of evolution of cell traits associated to conflict escalation within multicellular aggregates.     

Synrhabdosome life cycles

Synrhabdosomes are monospecific colonies of colonies of graptolites. Their mode of formation is unknown. We draw attention to a type of colony formation found in two genera of colonial rotifers and suggest that these represent plausible models for the life cycle of graptolite synrhabdosomes.     

Relationships among development, ecology, and morphology in the evolution of Echinoderm larvae and life cycles

The evolution of life cycles involves transitions between discrete states in one or more of the characters that comprise a developmental pattern. In this paper, we examine three of the major life cycle characters and the states for these characters. Using examples from echinoderms, we discuss the evolutionary transitions that have occurred in the type of morphogenesis, developmental habitat, and mode of nutrition during development. We evaluate the functional requirements associated with these transitions to infer the likelihood (frequency or rapidity) of change in a given character and of biases in the polarity of character state transitions. Using comparisons of closely related species, we evaluate the change between states in one character for dependence on the state of, or correlated changes in, other characters. Based on our analysis of congeneric species that differ in developmental habitat, we conclude that the transition between pelagic and benthic development is an ecological change that is independent of changes in morphogenesis and should be reversible. In contrast, the transition from feeding to nonfeeding development has been considered to be irreversible because it involves marked changes in larval morphology. We re-examine the transition between different modes of larval nutrition in light of recent studies that show that there exists a continuum of nutritional strategies between planktotrophy and lecithotrophy. This continuum is largely determined by variation in maternal investment and does not involve alterations in larval morphology. We suggest that the boundary between planktotrophy and lecithotrophy is frequently crossed and that this transition is reversible. Ecological changes represent the crossing of a functional threshold. Only after crossing the threshold, do larvae experience qualitatively different selective pressures that can lead to subsequent changes in morphology and development. Two different changes have occurred in the type of morphogenesis: the simplification of larval morphology that is associated with obligate (nonfeeding) lecithotrophy and the loss of the larval body plan in the evolution from indirect to direct development. It is the modification of morphology independent of the ecological changes that requires alterations in developmental processes, constrains evolutionary options, imposes irreversibility, and establishes the discrete nature of larval patterns in marine invertebrates.     

How rapidly does the human mitochondrial genome evolve?

The results of an empirical nucleotide-sequencing approach indicate that the evolution of the human mitochondrial noncoding D-loop is both more rapid and more complex than is revealed by standard phylogenetic approaches. The nucleotide sequence of the D-loop region of the mitochondrial genome was determined for 45 members of a large matrilineal Leber hereditary optic neuropathy pedigree. Two germ-line mutations have arisen in members of one branch of the family, thereby leading to triplasmic descendants with three mitochondrial genotypes. Segregation toward the homoplasmic state can occur within a single generation in some of these descendants, a result that suggests rapid fixation of mitochondrial mutations as a result of developmental bottlenecking. However, slow segregation was observed in other offspring, and therefore no single or simple pattern of segregation can be generalized from the available data. Evidence for rare mtDNA recombination within the D-loop was obtained for one family member. In addition to these germ-line mutations, a somatic mutation was found in the D-loop of one family member. When this genealogical approach was applied to the nucleotide sequences of mitochondrial coding regions, the results again indicated a very rapid rate of evolution.     

Morphoprocess and life cycles of organisms

In developing the ideas of V.N. Beklemishev about an organism as a form, existing in a process of determined transformation and matter/energy exchange, we consider different aspects of the term "morphoprocess" and introduce corresponding additional terms. Momentary morphoprocess characterizes an organism in the given moment of time. This term reflects a constancy of the form ("momentary form"), where the existence of an organism can be imagined as a sequence of "momentary forms". "First derivative" of this momentary characteristic is particular morphoprocess--an organism from its origin to fission/division or death. Compound particular morphoprocess is a determined and reiterating sequence of different particular morphoprocesses. And, at last, general morphoprocess--a "second derivative" of momentary morphoprocess--is rhythmical reiteration of a particular morphoprocess on the long-term scale, an ancestors/descendants lineage. To describe consecutive changes in this material system, the terms ontogenesis and life cycle are used. Ontogenesis characterizes a sequence of the morpho-functional changes of an individual organism during its life, whereas life cycle reflects a sequence of changes during one complete segment of the general morphoprocess represented by a single or several particular morphoprocesses. We also discuss morphoprocess uniformity along with the phase nature of morphoprocesses, both particular and compound particular ones.     

Long life cycles in insects

Long life cycles covering more than one year are known for all orders of insects. There are different mechanisms of prolongation of the life cycle: (1) slow larval development; (2) prolongation of the adult stage with several reproduction periods; (3) prolongation of diapause; (4) combination of these mechanisms in one life cycle. Lasting suboptimal conditions (such as low temperature, low quality of food or instability of food resources, natural enemies, etc.) tend to prolong life cycles of all individuals in a population. In this case, the larvae feed and develop for longer than a year, and the active periods are interrupted by dormancy periods. The nature of this dormancy is unknown: in some cases it appears to be simple quiescence, in others it has been experimentally shown to be a true diapause. Induction and termination of these repeated dormancy states are controlled by different environmental cues, the day-length being the principal one as in the case of the annual diapause. The long life cycles resulting from prolonged adult lifespan were experimentally studied mainly in beetles and true bugs. The possibility of repeated diapause and several periods of reproductive activity is related to the fact that the adults remain sensitive to day length, which is the main environmental cue controlling their alternative physiological states (reproduction vs. diapause). Habitats with unpredictable environmental changes stimulate some individuals in a population to extend their life cycles by prolonged diapause. The properties of this diapause are poorly understood, but results of studies of a few species suggest that this physiological state differs from the true annual diapause in deeper suppression of metabolism. Induction and intensity of prolonged diapause in some species appear to be genetically controlled, so that the duration of prolonged diapause varies among individuals in a group, even that of sibles reared under identical conditions. Thus, long life cycles are realized due to the ability of insects to interrupt activity repeatedly and enter dormancy. This provides high resistance to various environmental factors. Regardless of the nature of this dormancy (quiescence, annual or prolonged diapause, or other forms) and the life cycle duration, the adults always appear synchronously after dormancy in the nature. The only feasible explanation of this is the presence of a special synchronizing mechanism, most likely both exo- and endogenous, since the adults appear not only synchronously but also in the period best suited for reproduction. As a whole, the long life cycles resulting from various structural modifications of the annual life cycle, are typical of the species living under stable suboptimal conditions when the pressure of individual environmental factors is close to the tolerance limits of the species, even though it represents its norm of existence. Such life cycles are also typical of the insects living in unstable environments with unpredictable variability of conditions, those developing in cones and galls, feeding on flowers, seeds, or fruits with limited periods of availability, those associated with the plant species with irregular patterns of blossoming and fruiting, and those consuming low-quality food or depending on unpredictable food sources (e.g., predators or parasites). Long cycles are more common in: (1) insect species at high latitudes and mountain landscapes where the vegetation season is short and unstable; (2) species living in deserts or arid areas where precipitation is unstable and often insufficient for survival of food plants; (3) inhabitants of cold and temporary water bodies that are not filled with water every year. At the same time, long life cycles sometimes occur in insects from other climatic zones as well. It is also important to note that while there is a large body of literature dealing with the long life cycles in insects, it mostly focuses on external aspects of the phenomenon. Experimental studies are needed to understand this phenomenon, first of all the nature of dormancy and mechanisms of synchronization of adult emergence.     

How mechanisms of habitat preference evolve and promote divergence with gene flow

Habitat preference may promote adaptive divergence and speciation, yet the conditions under which this is likely are insufficiently explored. We use individual‐based simulations to study the evolution and consequence of habitat preference during divergence with gene flow, considering four different underlying genetically based behavioural mechanisms: natal habitat imprinting, phenotype‐dependent, competition‐dependent and direct genetic habitat preference. We find that the evolution of habitat preference generally requires initially high dispersal, is facilitated by asymmetry in population sizes between habitats, and is hindered by an increasing number of underlying genetic loci. Moreover, the probability of habitat preference to emerge and promote divergence differs greatly among the underlying mechanisms. Natal habitat imprinting evolves most easily and can allow full divergence in parameter ranges where no divergence is possible in the absence of habitat preference. The reason is that imprinting represents a one‐allele mechanism of assortative mating linking dispersal behaviour very effectively to local selection. At the other extreme, direct genetic habitat preference, a two‐allele mechanism, evolves under restricted conditions only, and even then facilitates divergence weakly. Overall, our results indicate that habitat preference can be a strong reproductive barrier promoting divergence with gene flow, but that this is highly contingent on the underlying preference mechanism.