COMMENT ON MARDEN (2013): “REANALYSIS AND EXPERIMENTAL EVIDENCE INDICATE THAT THE EARLIEST TRACE FOSSIL OF A WINGED INSECT WAS A SURFACE SKIMMING NEOPTERAN”

Marden's (2013) reanalysis of Knecht et al. (2011) suggesting that specimen SEMC‐F97 is the result of the skimming behavior of a neopteran insect and, more importantly, fossil evidence of “… surface skimming as a precursor to the evolution of flight in insects” (Marden 2013) is found to be deficient on three fronts: (1) the principal specimen was never viewed firsthand which led to significant morphological misinterpretations; (2) poorly designed and executed neoichnological experiments led to incredulous results; and (3) the assumption that this specimen is fossil evidence supporting the surface skimming hypothesis of the origin of insect flight despite the fact that since its induction into the literature that hypothesis has been refuted based on significant paleontological, phylogenetic, genetic, and developmental evidence.     

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.     

COMMENT ON GOHLI ET AL. (2013): “DOES PROMISCUITY EXPLAIN DIFFERENCES IN LEVELS OF GENETIC DIVERSITY ACROSS PASSERINE BIRDS?”

Gohli et al. (2013) report a positive relationship between genetic diversity and promiscuity across passerine birds, and suggest that female promiscuity acts as a form of balancing selection, maintaining differences in genetic variation across species. This is an interesting hypothesis, but the enormous variation in genetic diversity present within species is not taken into account in their analyses. This, combined with a small sample size at several levels, makes the relationship between genetic diversity and promiscuity very difficult to interpret. Demonstrating that species‐level differences in genetic diversity (if they occur at all) are affected by promiscuity would require a far more comprehensive study than is presently possible.     

APPLICATION OF PHYLOGENETIC PRINCIPLES TO TESTING EVOLUTIONARY SCENARIOS: A COMMENT ON KACZMARSKA ET AL. “MOLECULAR PHYLOGENY OF SELECTED MEMBERS OF THE ORDER THALASSIOSIRALES (BACILLARIOPHYTA) AND EVOLUTION OF THE FULTOPORTULA”1

While rigorous techniques have usually been used to generate phylogenetic trees from molecular data, morphological analysis has sometimes been more informal. A recent example was a study of the evolution of the fultoportula in the diatom order Thalassiosirales ( Kaczmarska et al. 2006 ). Phylogeny was inferred using modern phylogenetic principles applied to nuclear SSU rDNA sequences, but inferences about morphological character evolution were made using noncanonical reasoning and evolutionary scenario building. The preferred hypothesis posited that marginal fultoportulae evolved from the marginal ridge of Lithodesmiales. A related hypothesis suggested that fultoportulae in the valve center were not homologous with those near the valve margin. Shared symplesiomorphies, shared homoplasies, gaps in the fossil record, and subtle morphological differences between central‐ and marginal‐area fultoportulae were offered as the primary evidence for these scenarios. The literature has demonstrated such arguments to be either irrelevant or logically weaker than inferences made under the tests of similarity, conjunction, and congruence. Five prior hypotheses about the origin and evolution of the fultoportula were examined in this study using these tests. The hypothesis that the areola evolved into the multistrutted process, which evolved into the fultoportula, was best supported.     

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.     

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.     

ORIGIN AND RECENT ENDEMIC DIVERGENCE OF A CASPIAN MYSIS SPECIES FLOCK WITH AFFINITIES TO THE “GLACIAL RELICT” CRUSTACEANS IN BOREAL LAKES

Aspects of the evolution of intralacustrine species flocks and of the origin of the Arctic or “glacial-relict” zoogeographical element in Eurasian inland waters were elucidated in an allozyme study of the crustacean genus Mysis. This element, of supposedly northern marine ancestry, is represented by vicarious taxa in the deeper parts of the Caspian Sea (an enclosed ancient basin) and in young boreal lakes. The three endemic Caspian Mysis species studied are very close genetically (Nei's D = 0.06), which suggests a recent intrabasin radiation and rapid morphological divergence. This is in contrast to the pattern in postglacial Holarctic boreal lakes, where the Mysis relicta group is represented by a set of morphologically uniform but probably much older sibling species (D = 0.3–0.6). The results provide a parallel to those on the recent diversification of some fish species flocks in ancient freshwater lakes. The situation is, however, unusual in that the Caspian sympatric Mysis flock is pelagic, and conditions promoting speciation through allopatric isolation or spatial segregation by trophic substrate specialization seem implausible. The monophyletic Caspian Mysis clade shows a relatively strong divergence from both the northern lacustrine and the Arctic marine congeners (D = 0.6–1.0); the phylogenetic branching order of these three zoogeographical groups is not conclusively resolved. The results contradict the prevailing hypothesis of a recent Pleistocene origin of the Caspian Arctic element by invasion from Eastern European continental proglacial lakes that drained south to the Caspian basin during the glacial maxima and served as refugia for the boreal lacustrine taxa.     

THE SMALLEST DINOFLAGELLATE GENOME IS YET TO BE FOUND: A COMMENT ON LAJEUNESSE ET AL. “SYMBIODINIUM (PYRRHOPHYTA) GENOME SIZES (DNA CONTENT) ARE SMALLEST AMONG DINOFLAGELLATES”1

LaJeunessse and colleagues (LaJeunesse et al. 2005) have recently documented small genome sizes of Symbiodinium and concluded that Symbiodinium is a dinoflagellate lineage with the smallest genome. The conclusion is inconsistent with recent discoveries of picoplanktonic dinoflagellates. The search for the smallest genome and the effort to understand the evolutionary history of dinoflagellate genome should be an area of research in the years to come, which can be greatly aided by an understanding on the current hypotheses regarding mechanisms of genome size evolution. Even the smallest dinoflagellate genome documented to date is too large to be sequenced with current technology, but sequencing of chromosomes or expressed genes of key representative species is feasible and can be very insightful for understanding genome composition and function in this important lineage of eukaryotes.     

THE ROLE OF PHYLLOTACTIC PATTERN AS A “DEVELOPMENTAL CONSTRAINT” ON THE INTERCEPTION OF LIGHT BY LEAF SURFACES

Computer simulations of model plants are used to assess the influence of leaf shape, size, and pattern of arrangement (= phyllotaxy) on the direct solar radiation intercepted by leaf surfaces. Changes in phyllotaxy significantly influence light interception (and, by inference, net assimilation rate) for rosette growth habits. However, changes in leaf shape and orientation and in stem length can compensate for the negative effects of leaf overlap produced by phyllotactic patterns. Phyllotaxy is viewed as a developmental limiting factor in photobiology that may necessitate compensatory changes in other morphological features not directly controlled by patterns of leaf initiation. This distinguishes it from functioning as a “developmental constraint” sensu stricto and may provide a paradigm for other features in plant evolution.     

STRIP-LIKE COVERING REVEALED ON “WALL-LESS”ASTEROMONAS (CHLOROPHYCEAE)1

Asteromonas Artari previously described as a wall-less biflagellated unicell exhibits a strip-like cell covering on some cells. Both tannic acid fixation and freeze-etch reveal the covering of electron opaque strips outside the plasmalemma. Cross striations are apparent in the surface strips. Golgi vesicles contain material presumed to be precursor components of the covering strips. The crystalline pattern of the freeze-etched material is similar to that reported for other, walled volvocalean algae.     

THE “PARADOX” DIATOM BACILLARIA PAXILLIFER (BACILLARIOPHYTA) REVISITED1

Defined by its unique colonial locomotion, Bacillaria paxillifer (O. F. Müll.) Hendey was recognized as a single, pandemic species by many phycologists. However, reinvestigation of colonies from different habitats revealed three distinct groups: (A) brackish/freshwater, (B) marine littoral, and (C) marine planktonic taxa. Groups differed in colony and cell form, raphe flanges (RFs), shape and position of transapical ribs (Tr's), and morphogenesis. Linear‐shaped species were restricted to group A: Tr's thickened principally to the interior. Lanceolate forms were confined to groups B and C: valve formation proceeded from an internal base layer to the exterior. The planktonic species differed in the shape of its raphe slit, and the transformation of girdle bands (GBs) into “winglets.” Taxa also differed in chloroplast shape and number. All species formed motile colonies. Siblings adhered via elastic fibrils secreted through their raphe. Raphe ribs were held in position by siliceous clamps (fibulae), anchored in an extra pair of axial ribs (fibular ribs) parallel to the raphe ribs. This raphe system resembled that of Cylindrotheca rather than the “canal raphe” of Nitzschia. Many valves were asymmetric along the apical axis due to protruding RFs shuttling in a 1:1 ratio within a colony, but raphe slits were mirror images, as were the growth direction of fibulae and position of plastids, with pyrenoids tilted in the same direction. Species possessed four open GBs per epitheca; the third band invariably bore an internal, organic ridge to aid in adhesion of the plasmalemma during cleavage. The results suggested that these taxa are a natural phylogenetic group, requiring precise determination of their taxonomic position.     

COST OF GLANDULAR TRICHOMES,A “RESISTANCE” CHARACTER IN DATURA WRIGHTII REGEL (SOLANACEAE)

Models regarding the evolution of plant resistance to herbivory often assume that the primary mechanism maintaining resistance polymorphisms is the balance between benefits of increased resistance to herbivores and costs associated with the production of a resistance character. However, rarely has it been demonstrated that genetically based resistance traits are costly. Here, we document costs associated with the production of glandular trichomes, a resistance character in Datura wrightii that is predominantly under the control of a single gene of large effect. In the absence of herbivores, plants with glandular trichomes (sticky) produced 45% fewer viable seeds than plants with nonglandular trichomes (velvety). Although both plant types flowered with similar frequency, sticky plants matured fewer capsules and fewer of their seeds germinated. The fitness difference between the types in herbivore-free conditions was not mitigated by the addition of water, a potentially limiting resource for sticky plants. Under herbivore pressure, there was no significant fitness difference between the types, although the fitness of velvety plants was still higher than that of sticky plants. This occurred even though velvety plants sustained more herbivore damage than sticky plants and were more likely to be attacked by most herbivore species present. The fitness difference between the plant types was especially reduced when herbivore-attacked plants were watered, which indicates that sticky plants may have higher tolerance for damage than velvety plants when supplied with a potentially limiting resource. Yet, the maintenance of a fitness deficit (albeit small and nonsignificant) for sticky plants when attacked by herbivores indicates no net benefit associated with the production of glandular trichomes in this first year of our study. These results add to our current understanding that herbivore resistance characters can be costly and raise the question of how this genetic polymorphism is maintained in wild populations.     

EXPLAINING THE SOCIOBIOLOGY OF PYOVERDIN PRODUCING PSEUDOMONAS: A COMMENT ON ZHANG AND RAINEY (2013)

Over the past decade, there has been enormous interest in understanding the great diversity of microbial cooperative behaviors, including communication, group‐based swarming, fruiting‐body formation, and the secretion of group‐beneficial enzymes and food‐scavenging molecules. Zhang and Rainey, henceforth Z&R, recently contended that sociomicrobiologists have been overzealous in their casting of microbes as inherently social organisms, and too hasty in interpreting microbial behaviors in a social evolutionary framework. This challenge accompanied a set of experiments in which they revisited one of the best‐studied social behaviors in bacteria—the production of diffusible, sharable iron‐scavenging siderophore molecules. Z&R posit that their findings challenge the view that siderophore production is a cooperative trait. Here, we demonstrate that their arguments are flawed, and stem from both technical mistakes and misunderstandings of social evolution theory.