The chiasma-inducing pheromone of locusts

Evolution and Plasticity of Genome-Wide Meiotic Recombination Rates

Sex, as well as meiotic recombination between homologous chromosomes, is nearly ubiquitous among eukaryotes. In those species that use it, recombination is important for chromosome segregation during gamete production, and thus for fertility. Strikingly, although in most species only one crossover event per chromosome is required to ensure proper segregation, recombination rates vary considerably above this minimum and show variation within and among species. However, whether this variation in recombination is adaptive or neutral and what might shape it remain unclear. Empirical studies and theory support the idea that recombination is generally beneficial but can also have costs. Here, we review variation in genome-wide recombination rates, explore what might cause this, and discuss what is known about its mechanistic basis. We end by discussing the environmental sensitivity of meiosis and recombination rates, how these features may relate to adaptation, and their implications for a broader understanding of recombination rate evolution. Expected final online publication date for the Annual Review of Genetics, Volume 55 is November 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Connecting theory and data to understand recombination rate evolution

Meiotic recombination is necessary for successful gametogenesis in most sexually reproducing organisms and is a fundamental genomic parameter, influencing the efficacy of selection and the fate of new mutations. The molecular and evolutionary functions of recombination should impose strong selective constraints on the range of recombination rates. Yet, variation in recombination rate is observed on a variety of genomic and evolutionary scales. In the past decade, empirical studies have described variation in recombination rate within genomes, between individuals, between sexes, between populations and between species. At the same time, theoretical work has provided an increasingly detailed picture of the evolutionary advantages to recombination. Perhaps surprisingly, the causes of natural variation in recombination rate remain poorly understood. We argue that empirical and theoretical approaches to understand the evolution of recombination have proceeded largely independently of each other. Most models that address the evolution of recombination rate were created to explain the evolutionary advantage of recombination rather than quantitative differences in rate among individuals. Conversely, most empirical studies aim to describe variation in recombination rate, rather than to test evolutionary hypotheses. In this Perspective, we argue that efforts to integrate the rich bodies of empirical and theoretical work on recombination rate are crucial to moving this field forward. We provide new directions for the development of theory and the production of data that will jointly close this gap.This article is part of the themed issue 'Evolutionary causes and consequences of recombination rate variation in sexual organisms'.

Meiosis evolves: adaptation to external and internal environments

306 I. 306 II. 307 III. 312 IV. 317 V. 318 319 References 319 SUMMARY: Meiosis is essential for the fertility of most eukaryotes and its structures and progression are conserved across kingdoms. Yet many of its core proteins show evidence of rapid or adaptive evolution. What drives the evolution of meiosis proteins? How can constrained meiotic processes be modified in response to challenges without compromising their essential functions? In surveying the literature, we found evidence of two especially potent challenges to meiotic chromosome segregation that probably necessitate adaptive evolutionary responses: whole-genome duplication and abiotic environment, especially temperature. Evolutionary solutions to both kinds of challenge are likely to involve modification of homologous recombination and synapsis, probably via adjustments of core structural components important in meiosis I. Synthesizing these findings with broader patterns of meiosis gene evolution suggests that the structural components of meiosis coevolve as adaptive modules that may change in primary sequence and function while maintaining three-dimensional structures and protein interactions. The often sharp divergence of these genes among species probably reflects periodic modification of entire multiprotein complexes driven by genomic or environmental changes. We suggest that the pressures that cause meiosis to evolve to maintain fertility may cause pleiotropic alterations of global crossover rates. We highlight several important areas for future research.

The dark-color inducing neuropeptide, [His(7)]-corazonin, causes a shift in morphometic characteristics towards the gregarious phase in isolated-reared (solitarious) Locusta migratoria

The neurohormone, [His(7)]-corazonin is known to induce dark color in the cuticle and epidermis of Locusta migratoria. In the present study, we examined the effects of this hormone on development and morphometrics in two strains, albino and normal, of this locust under isolated conditions. Injection of [His(7)]-corazonin induced dark color in both strains. In either strain, [His(7)]-corazonin injected at the second and third instars did not affect duration of nymphal development or the number of nymphal instars. The shape of the pronotum was more convex in isolated-reared animals than in crowd-reared ones, and injection of [His(7)]-corazonin caused isolated-reared animals to develop a less convex pronotum in the normal strain injected at a high dose (1 nmolx2) but not in the albino strain injected at a low dose (50 pmolx2). [His(7)]-corazonin injected into isolated-reared nymphs caused a shift in classical morphometric ratios (F/C and E/F; F=length of the hind femur, C=maximum width of the head, E=length of the fore wings) towards values typical for crowd-reared (gregarious) individuals of the two strains. This study demonstrated for the first time that [His(7)]-corazonin affected morphometric characteristics in L. migratoria.

Effects of [His(7)]-corazonin on the phase state of isolated-reared (solitarious) desert locusts, Schistocerca gregaria

[His(7)]-corazonin is a neuropeptide produced in the pars lateralis of the brain. It is stored in the corpora cardiaca and probably released from there. The only well-documented effect in locusts is increased melanization of the cuticle. We investigated whether this hormone might also be causally related to changes in behavior and morphometrics that, like melanization, occur during crowding-induced gregarization. Solitary fourth-instar nymphs of Schistocerca gregaria (Forsk.) were injected thrice with 1 nmol [His(7)]-corazonin. After molting to the 5th stadium their behavioral phase state was measured in an arena assay and analyzed using multiple logistic regression analysis. The hormone was found not to induce behavioral phase changes. Upon reaching adulthood, morphometrics shifts occurred towards values typical for crowd-reared and regregarized animals. Our results thus indicate that [His(7)]-corazonin is not involved in behavioral gregarization but may participate in morphometrical phase change.

A comparison of phase-related shifts in behavior and morphometrics of an albino strain, deficient in [His(7)]-corazonin, and a normally colored Locusta migratoria strain

The albino Okinawa strain of Locusta migratoria is deficient in the neurohormone [His(7)]-corazonin. This peptide induces darkening of the cuticle, one of the typical features of gregarious locusts. As part of a broader study on the possible role of [His(7)]-corazonin in phase transition, we explored whether corazonin-deficiency might be associated with differences in behavior and morphometrics between albino and normal phenotypes of L. migratoria. Using a modification of the logistic-regression assay of behavioral phase state previously derived for Schistocerca gregaria, we found that there were strain dependent behavioral differences between crowd-reared nymphs of the albino Okinawa and the normally colored African strain, with no evidence of the albino strain being obligatorily solitarious. However, upon isolation, a shift towards more solitarious behavior occurs in both strains, even more profoundly in the Okinawa albinos. A shift could also be recorded in morphometrics. The conclusion is that the albino strain, although showing some solitarious features even when crowd-reared, is not, as has been suggested, obligatory solitarious and, as a consequence, the complete absence of corazonin is not sufficient to bring about the solitarious state.

The physiology of locust phase polymorphism: an update

The considerable progress made between 1990 and 1997 in locust phase-related research and in understanding the physiology of locust phase polymorphism is reviewed. The traits of locust phases are discussed and it is concluded that there are distinct strain-dependent differences in phase characteristics and their amplitudes even in the same species. Despite some advances, no major break-through was achieved in the putative endocrine control of locust phase polymorphism. Phase-dependent differences in adipokinesis, flight fuels and migration of adult locusts, as well as novel methods in studying aggregation behaviour and activity of hoppers and adults, opened new lines in research of the physiology of locust phase polymorphism. Marked advances were made in phase-related locust pheromone research, revealing, in Schistocerca gregaria, differences between the pheromonal system of the hoppers and that of the adults. These systems turned out to be more complex than previously assumed. Phenylacetonitrile, produced by sexually mature adult males, serving both as an attractant and a mutration-accelerating factor, was identified as the major compound of the adult pheromonal system in S. gregaria. A new aspect of transmission of phase characteristics from parent to progeny through the foam (froth) of the egg pod was revealed. Effects of some plant substances on locust phases were reported. However, no research has yet been published on the aspects of molecular biology of locust phase polymorphism.