Genetic parsimony: a factor in the evolution of complexity, order and emergence

Two conjectures, drawn from Gregory Chaitin's Algorithmic Information Theory, are examined with respect to the relationship between an algorithm and its product; in particular his finding that, where an algorithm is minimal, its length provides a measure of the complexity of the product. Algorithmic complexity is considered from the perspective of the relationship between genotype and phenotype, which Chaitin suggests is analogous to other algorithm-product systems. The first conjecture is that the genome is a minimal set of algorithms for the phenotype. Evidence is presented for a factor, here termed 'genetic parsimony', which is thought to have helped minimize the growth of genome size during evolution. Species that depend on rapid replication, such as prokaryotes which are generally r -selected are more likely to have small genomes, while the K -strategists accumulate introns and have large genomes. The second conjecture is that genome size could provide a measure of organism complexity. A surrogate index for coding DNA is in agreement with an established phenotypic index (number of cell types), in exhibiting an evolutionary trend of increasing organism complexity over time. Evidence for genetic parsimony indicates that simplicity in coding has been selected, and is responsible for phenotypic order. It is proposed that order evolved because order in the phenotype can be encoded more economically than disorder. Thus order arises due to selection for genetic parsimony, as does the evolution of other 'emergent' properties.  © 2006 The Linnean Society of London, Biological Journal of the Linnean Society , 2006, 88 , 295–308.     

Experimental evidence that source genetic variation drives pathogen emergence

A pathogen can readily mutate to infect new host types, but this does not guarantee successful establishment in the new habitat. What factors, then, dictate emergence success? One possibility is that the pathogen population cannot sustain itself on the new host type (i.e. host is a sink), but migration from a source population allows adaptive sustainability and eventual emergence by delivering beneficial mutations sampled from the source''s standing genetic variation. This idea is relevant regardless of whether the sink host is truly novel (host shift) or whether the sink is an existing or related, similar host population thriving under conditions unfavourable to pathogen persistence (range expansion). We predicted that sink adaptation should occur faster under range expansion than during a host shift owing to the effects of source genetic variation on pathogen adaptability in the sink. Under range expansion, source migration should benefit emergence in the sink because selection acting on source and sink populations is likely to be congruent. By contrast, during host shifts, source migration is likely to disrupt emergence in the sink owing to uncorrelated selection or performance tradeoffs across host types. We tested this hypothesis by evolving bacteriophage populations on novel host bacteria under sink conditions, while manipulating emergence via host shift versus range expansion. Controls examined sink adaptation when unevolved founding genotypes served as migrants. As predicted, adaptability was fastest under range expansion, and controls did not adapt. Large, similar and similarly timed increases in fitness were observed in the host-shift populations, despite declines in mean fitness of immigrants through time. These results suggest that source populations are the origin of mutations that drive adaptive emergence at the edge of a pathogen''s ecological or geographical range.     

Evolution of the population structure of Venturia inaequalis,the apple scab fungus,associated with the domestication of its host

Evaluating the impact of plant domestication on the population structure of the associated pathogens provides an opportunity to increase our understanding of how and why diseases emerge. Here, we investigated the evolution of the population structure of the apple scab fungus Venturia inaequalis in response to the domestication of its host. Inferences were drawn from multilocus microsatellite data obtained from samples collected on (i) the Central Asian Malus sieversii, the main progenitor of apple, (ii) the European crabapple, Malus sylvestris, a secondary progenitor of apple, and (iii) the cultivated apple, Malus×domestica, in orchards from Europe and Central Asia. Using clustering methods, we identified three distinct populations: (i) a large European population on domesticated and wild apples, (ii) a large Central Asian population on domesticated and wild apples in urban and agricultural areas, and (iii) a more geographically restricted population in M. sieversii forests growing in the eastern mountains of Kazakhstan. Unique allele richness and divergence time estimates supported a host‐tracking co‐evolutionary scenario in which this latter population represents a relict of the ancestral populations from which current populations found in human‐managed habitats were derived. Our analyses indicated that the domestication of apple induced a significant change in the genetic differentiation of populations of V. inaequalis in its centre of origin, but had little impact on its population dynamics and mating system. We discuss how the structure of the apple‐based agrosystem may have restricted changes in the population structure of the fungus in response to the domestication of its host.     

Circadian rhythms of adult emergence and activity but not eclosion in males of the parasitic wasp Nasonia vitripennis

An endogenous circadian system is responsible for the rhythms observed in many physiological and behavioural traits in most organisms. In insects, the circadian system controls the periodicity of eclosion, egg-laying, locomotor and mating activity. The parasitoid wasp Nasonia vitripennis has been extensively used to study the role of the circadian system in photoperiodism. In this study, behavioural activities expected to be under the control of the endogenous circadian system were characterized in Nasonia. Male emergence from the host puparium is rhythmic under light-darkness conditions while eclosion from the own pupal integument is not rhythmic but continuous. Following entrainment in light-dark conditions, males show robust free-running circadian activity rhythms with a period (τ, tau) of approximately 25.6 h in constant darkness. While the endogenous circadian system is enough to trigger male emergence in Nasonia, light seems to have a modulatory effect: when present it induces more males to emerge. Our results add to the understanding of chronobiological phenotypes in insects and provide a basis towards the molecular characterization of the endogenous circadian system in Nasonia.     

Developmental disorders in buds and needles of mature Norway spruce, Picea abies (L.) Karst., in relation to needle boron concentrations

Dieback of the terminal shoot and consequently bushy growth induced by boron deficiency have been reported widely throughout the world in several tree species. Recently, similar growth damage was documented in half of the young spruce stands in eastern Finland. To clarify the role of B deficiency, the light microscopic structure of emerging buds and of developing and previous-year needles of mature Norway spruce (Picea abies L. Karst.) from damaged (D stand), partly damaged (PD stand) and healthy (H stand) stands were analysed. The samples, on which needle nutrient concentrations were also determined, were taken seven times between early spring (April) and early winter (November). Cell death characterized by precipitation of the cell content, possibly due to the release of tannins after membrane rupture, was seen in the apex of emerging buds, and this led to fatal damage in about half of the buds in the trees from the D stand, where the needle B concentration was well below the deficiency level of 4–5 mg kg⁻¹. Furthermore, an increase in living cells that accumulated tannins in the vacuoles, which is a common stress and/or defense reaction, was found in the primordial shoots of buds and in the differentiating needles in the PD and D stands. The increase in the areas of the central cylinder and of the xylem found in the needles indicate structural plasticity during needle differentiation to drought. The time frame for bud emergence from late May up to mid-September means that an adequate B supply is necessary throughout the summer in order to avoid fatal bud damage and thus bushy growth of the trees.     

Impact of the Timing of Male Emergence on Mating Capacity of Males in Trichogramma evanescens Westwood

In species with highly structured population, such as Trichogramma spp., mating occurs mostly at emergence on the patch and early emerging males have an advantage, as they are present when the first females emerge. However, early emergence of male could also enable males to mature sexually before the emergence of females or enhance their capacity to induce higher receptivity in females. We measured time of emergence of male and female Trichogramma evanescens Westwood (Hymenoptera: Trichogrammatidae) from hosts that were parasitized within a 30 min period. We also measured the effect of male age on their capacity to mate, and the ability of inseminated females to produce daughters. To verify if early emerging males induced higher receptivity in females, we observed the females mate choice between males of different ages. Our results indicated that the mean time of emergence between males and females was 29 min for individuals that develop in 9 days and 10 min for individuals that develop in 10 days. The protandry observed in this species could be the result of either a faster development of males or a male first strategy by the ovipositing female. In T. evanescens, protandry does not permit males to mature sexually nor to induce higher receptivity in females. The main advantage of protandry for males thus appears to be early access to females as they emerge.     

A butenolide, isolated from smoke, can overcome the detrimental effects of extreme temperatures during tomato seed germination

The butenolide, 3-methyl-2H-furo[2, 3-c]pyran-2-one, is an highly active compound isolated from plant-derived smoke. This compound is known to stimulate seed germination in a wide range of plants akin to smoke or aqueous extracts of smoke. The present study attempted to elucidate the role of the butenolide in overcoming detrimental effects of low and high temperatures on tomato seed germination and seedling growth. The germination percentage followed a parabolic curve for temperatures ranging from 10 to 40°C, with 25°C being the optimum for all treatments. Control seeds showed radicle emergence at two extreme temperatures (10 and 40°C) and seedlings failed to develop further, even upon prolonged incubation. By comparison the butenolide-treated seeds grew into phenotypically normal seedlings at these non-optimum temperatures. The smoke–water-treated seeds had an intermediate response as only a fraction of germinated seed developed into normal seedlings. Seedling vigour indices as well as seedling weight were significantly higher (p ≤ 0.05) for butenolide-treated seeds at all temperatures. Furthermore, seedlings developed in the presence of the butenolide had about a 1:1 correspondence between root and shoot length. Butenolide-treated seeds grew better than the control seeds in the temperature shift experiments. A gradual decline in the vigour index values was recorded with an increased duration of incubation at the extreme temperatures. Results of the present study are very important from an horticultural point of view as they indicate the potential use of the butenolide compound in restoring normal seed germination and seedling establishment in tomato below and above optimum temperatures.     

水分和温度对春玉米出苗速度和出苗率的影响

在东北地区中部开展春玉米春季水分胁迫和分期播种试验。试验在人工防雨棚内进行,设4个水分处理和3个播种期处理。结果表明,春玉米(Zea mays )播种出苗期间土壤含水量与出苗期和出苗率的关系均呈显著的二次函数关系。在田间持水量以下,耕层土壤湿度越大,玉米出苗越快,出苗率越高;干旱推迟出苗期,降低出苗率。气温低出苗慢,但温度对出苗率无明显影响。播种出苗期间0～20cm深平均土壤湿度（S）、土壤有效水量（H）和平均气温(T)对出苗速度的组合影响指标是：25 %>S>21%（或70mm >H>45mm）、T>18℃玉米出苗快;21% >S>18%(或45mm >H>35mm)、18℃>T>16.5℃ 玉米出苗比较快;18%>S>16%（或35mm >H>25mm）、16.5℃>T>15.5℃玉米出苗较慢;S<16%（或H< 25）、T<15.5℃出苗很慢,出苗期明显推迟。玉米出苗率主要取决于水分,0～20cm深土壤相对湿度在85%以上（或H>50mm）出苗率最高（95%以上）;相对湿度80%左右（或50>H>40mm）出苗率较高（90% 左右）;相对湿度70%以下(或H<40mm)出苗率明显降低, 60%以下(或H<30mm)为严重干旱,出苗率低至65%以下。