THE CONTRIBUTION OF GENE MOVEMENT TO THE “TWO RULES OF SPECIATION”

The two “rules of speciation”—the Large X‐effect and Haldane's rule—hold throughout the animal kingdom, but the underlying genetic mechanisms that cause them are still unclear. Two predominant explanations—the “dominance theory” and faster male evolution—both have some empirical support, suggesting that the genetic basis of these rules is likely multifarious. We revisit one historical explanation for these rules, based on dysfunctional genetic interactions involving genes recently moved between chromosomes. We suggest that gene movement specifically off or onto the X chromosome is another mechanism that could contribute to the two rules, especially as X chromosome movements can be subject to unique sex‐specific and sex chromosome specific consequences in hybrids. Our hypothesis is supported by patterns emerging from comparative genomic data, including a strong bias in interchromosomal gene movements involving the X and an overrepresentation of male reproductive functions among chromosomally relocated genes. In addition, our model indicates that the contribution of gene movement to the two rules in any specific group will depend upon key developmental and reproductive parameters that are taxon specific. We provide several testable predictions that can be used to assess the importance of gene movement as a contributor to these rules in the future.     

THE EVOLUTION OF PHENOTYPIC CORRELATIONS AND “DEVELOPMENTAL MEMORY”

Development introduces structured correlations among traits that may constrain or bias the distribution of phenotypes produced. Moreover, when suitable heritable variation exists, natural selection may alter such constraints and correlations, affecting the phenotypic variation available to subsequent selection. However, exactly how the distribution of phenotypes produced by complex developmental systems can be shaped by past selective environments is poorly understood. Here we investigate the evolution of a network of recurrent nonlinear ontogenetic interactions, such as a gene regulation network, in various selective scenarios. We find that evolved networks of this type can exhibit several phenomena that are familiar in cognitive learning systems. These include formation of a distributed associative memory that can “store” and “recall” multiple phenotypes that have been selected in the past, recreate complete adult phenotypic patterns accurately from partial or corrupted embryonic phenotypes, and “generalize” (by exploiting evolved developmental modules) to produce new combinations of phenotypic features. We show that these surprising behaviors follow from an equivalence between the action of natural selection on phenotypic correlations and associative learning, well‐understood in the context of neural networks. This helps to explain how development facilitates the evolution of high‐fitness phenotypes and how this ability changes over evolutionary time.     

THE ULTRASTRUCTURE OF SCENEDESMUS (CHLOROPHYCEAE). I. SPECIES WITH THE “RETICULATE” OR “WARTY” TYPE OF ORNAMENTAL LAYER1

The ultrastructure of the wall layers and ornamentative features of Scenedesmus pannonicus and S. longus are described using carefully correlated freeze-etched replicas, thin sections and scanning electron micrographs. The two species arc enclosed by different types of ornamented layers, S. pannonicus by the tightly filling, “warty” layer and S. longus by the loosely fitting, “reticulate” layer, held off the coenobium by 2 types of tubular propping spikelets and rosettes. The reticulate layer has an intricate substructure, especially when studied with freeze-etching. Its inner and outer surfaces appear different, as is its attachment to the 2 types of spikelets. Whole cells of S. longus subjected to acetolysis lack the cellulose wall and cytoplasm, but all other surface features survive, including the Trilaminar Sheath (TLS); this ornamentation cannot be “pectic.” The cellulose wall and ornamentation is unaffected by boiling water alone. Boiling in 6n NaOH removes the surface ornamentation, but the TLS and wall remain; the possibility that these features contain silica is discussed. The terminal spines of both species consist of closely packed spikelets enclosed within a skin of hexagonally-packed subunits. Similar subunits are seen in the propping spikelets of S. longus, and in the rows or “combs” of laterally fused spikelets of S. pannonicus. The warty layer of S. pannonicus is tightly appressed to the TLS except close to where the cells are joined, where it is suspended free. It is composed of a layer of globular subunits, and small indentations form the warts. Single, evenly distributed warts characterize the freely suspended sections of the warty layer, and the layer that encloses young coenobia soon after they have been formed: in contrast, the warts are clumped over the surface of older and larger colonies. Some of the single warts form characteristic double rows, but these latter remain single even on older cells. The surface structure of the warty layer, TLS, and plasmalemma are revealed by the freeze-etching process.     

COMPETITION BETWEEN SEGREGATION DISTORTERS: COEXISTENCE OF “SUPERIOR” AND “INFERIOR” HAPLOTYPES AT THE t COMPLEX

By means of population genetical models, we investigate the competition between sex-specific segregation distorters. Although the models are quite general, they are motivated by a specific example, the t complex of the house mouse. Some variants at this gene complex, the t haplotypes, distort Mendelian segregation in heterozygous males in their favor. The selective advantage at the gamete level is counterbalanced by strong negative fitness effects at the individual level (male sterility or even lethality in both sexes). A plethora of different t haplotypes has been found, both in the field and in the lab. Up to now, however, models have focused on the equilibrium frequency of a single t haplotype. In contrast, we explicitly model the competition between several t haplotypes. A deterministic model for a large, well-mixed population predicts a surprisingly high degree of polymorphism. Haplotypes with seemingly inferior fitness characteristics may easily coexist with “superior” haplotypes. For instance, a lethal haplotype with a low segregation ratio may stably coexist with a sterile haplotype with a high segregation ratio. Stable coexistence is even possible for haplotypes with a segregation disadvantage. A simple stochastic model shows that the same principles apply in the context of a structured metapopulation. Although counterintuitive at first sight, all our results can be explained by the fact that segregation distorters have an inherent advantage when they are rare. We conclude that fitness comparisons are not sufficient to predict the outcome of competition when selective forces are acting at different levels.     

PAIRED PREFERENCE “PLACEBO” TESTS WITH “IDENTICAL” STIMULI: DOES INTRODUCING GRADED PREFERENCE RESPONSES AFFECT THE FREQUENCY OF “NO PREFERENCE” RESPONSES?

In the "placebo" control condition for paired preference testing, "identical" stimuli are presented to consumers to determine the frequency of preference and "no preference" responses induced by the hidden demand characteristics of the testing condition. As a control for bias, induced by such hidden demand characteristics, these frequencies can be compared with the actual preference frequencies of the nonidentical test stimuli to be assessed for preference. It was hypothesized that the introduction of graded preference response options might reduce the frequency of "no preference" responses in the placebo condition. Using identical yogurt stimuli with related-sample (single-group) and independent-sample (multigroup) designs, this hypothesis was not confirmed.

PRACTICAL APPLICATIONS

The "placebo" condition in paired preference testing provides a way to control the tendency of consumers to give false preference. The statistical analysis can be improved by finding a protocol that maximizes the proportion of "no preference" responses in the placebo condition. This can be done by increasing the number of response options that imply no preference. Yet, there is sometimes a desire to increase the number of preference options by using graded responses for preference strength. Does this alter the dynamics of the ever-important placebo condition by reducing the number of "no preference" responses? This project found that any effect was minimal.     

THE GENETICS OF THE “BIOTYPES” OF THE RICE BROWN PLANTHOPPER,NILAPARVATA LUGENS

As the resistance of rice varieties to the brown planthopper is based on major genes it has been widely assumed that there is a gene for gene correspondence between resistance on the part of the plant and virulence on the part of the hopper. However, the mode of inheritance and response of the biotypes to selection, together with the previously reported wide variation within each biotype and large overlap between them in virulence, is all consistent with polygenic determination of virulence.

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Résumé Comme la résistance des variétés de riz à N. lugens est due à des gènes majeurs, on considère généralement qu'il y a une correspondance gène à gène entre la résistance de la plante et la virulence de l'insête. Cependant, le type d'héritabilité et la réponse des biotypes à la sélection, ainsi que la grande variabilité antérieurement signalée dans chaque biotype et l'important chevauchement de leur virulence, sont tous conformes à une détermination polygénique de la virulence.

     

A “NORMAL” DEVELOPMENT OF DENUDED EGGS OF THE STARFISH,ASTERINA PECTINIFERA*

Experimental conditions that allow “normal” development of starfish eggs stripped of the fertilization membrane are reported in this paper. Four kinds of intercellular relation are distinguished during the pre-hatching stages of these eggs. Cells from 2- to 8-cell stages are hardly related to each other, while those from 16- to 128-cell stages are bound loosely together. After the 8th division (about 5.5 hr after insemination at 21°C) cells adhere closely and cooperate with each other to perform morphogenetic movement of “blastulation”. This relation is taken over by that of a true multicellular system at about 10 hr after insemination. Closely after this, the function of cilia carries the embryo away from the substratum.     

TEMPERATURE-HUMIDITY “ORGAN” EXPERIMENTS WITH THREE SPECIES OF GRASSHOPPER BELONGING TO THE FAMILY OF ACRIDIDAE

Three ecologically well-differentiated species of grasshoppers were studied in a temperature-humidity organ. No statistically certain differences between the species were demonstrated.

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Zusammenfassung Drei ökologisch gut abgegrenzte Heuschreckenarten wurden untersucht. Die üblichen Methoden, an Hand derer man die Versuchsresultate der Temperatur-Feuchtigkeits-Orgel beurteilt, wurden kritisiert. Eine Arbeitsweise, statistisch analysierbare Werte zu erhalten, wird in dem vorliegenden Artikel aufgezeigt und angewandt. Zwischen den drei Arten wurden keine statistisch gesicherten Unterschiede erzielt. Die drei Arten zeigen nicht einmal eine Tendenz, sich in der Orgel auf eine Weise zu verteilen, die ihrere Verteilung im Verhältnis zum Temperaturfaktor in ihrer natürlichen Umgebung entspricht.

     

THE “BIOTYPES” OF THE RICE BROWN PLANTHOPPER,NILAPARVATA LUGENS

From previous studies, the biological nature of the so-called biotypes of N. lugens is obscure. Experiments on biotypes 1, 2 and 3 from the Philippines demonstrated that they lacked significant breeding barriers. Inbred biotype cultures showed considerable variation and overlap between each other in virulence. A field population from the Philippines was similarly variable and included individuals which could be attributed to different biotypes. It is concluded that the use of the term biotype in this species is biologically misleading and undesirable.

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Résumé La nature biologique de ce qui a été appelé des biotypes de Nilaparvata lugens, paraît obscure si l'on se réfère aux études antérieures. Des expériences sur les biotypes 1, 2 et 3 des Philippines révèlent qu'ils ne présentent pas de barrières sexuelles significatives. Les élevages consanguins de biotypes présentent une variabilité considérable, et les virulances des biotypes se chevauchent les unes les autres. De même une population récoltée dans les çhamps aux Philippines était hétérogène et comprenait des individus qui auraient pu être rattachés à différents biotypes. On peut en conclure que le terme de biotype est, pour cette espèce, inapproprié et indésirable.

     

HOST POPULATION STRUCTURE AND THE EVOLUTION OF VIRULENCE: A “LAW OF DIMINISHING RETURNS”

Structure in a population of host individuals, whether spatial or temporal, can have important effects on the transmission and evolutionary dynamics of its pathogens. One of these is to limit dispersal of pathogens and thus increase the amount of contact between a given pair or within a small group of host individuals. We introduce a “law of diminishing returns” that predicts an evolutionary decline of pathogen virulence whenever there are on average more possibilities of pathogen transmission between the same pair of hosts. Thus, the effect of repeated contact between hosts will be to shift the balance of any trade-off between virulence and transmissibility toward lower virulence.