Macroevolutionary Analyses Provide New Evidence of Phasmid Wings Evolution as a Reversible Process

Resource allocation strategies and mechanical constraints drive the diversification of stick and leaf insect eggs

The diversity of insect eggs is astounding but still largely unexplained. Here, we apply phylogenetic analyses to 208 species of stick and leaf insects, coupled with physiological measurements of metabolic rate and water loss on five species, to evaluate classes of factors that may drive egg morphological diversification: life history constraints, material costs, mechanical constraints, and ecological circumstances. We show support for all three classes, but egg size is primarily influenced by female body size and strongly trades off with egg number. Females that lay relatively fewer but larger eggs, which develop more slowly because of disproportionately low metabolic rates, also tend to bury or glue them in specific locations instead of simply dropping them from the foliage (ancestral state). This form of parental care then directly favors relatively elongated eggs, which may facilitate their placement and allow easier passage through the oviducts in slender species. In addition, flightless females display a higher reproductive output and consequently lay relatively more and larger eggs compared with flight-capable females. Surprisingly, local climatic conditions had only weak effects on egg traits. Overall, our results suggest that morphological diversification of stick insect eggs is driven by a complex web of causal relationships among traits, with dominant effects of resource allocation and oviposition strategies, and of mechanical constraints.

Parthenogenetic Stick Insects Exhibit Signatures of Preservation in the Molecular Architecture of Male Reproduction

After the loss of a trait, theory predicts that the molecular machinery underlying its phenotypic expression should decay. Yet, empirical evidence is contrasting. Here, we test the hypotheses that (i) the molecular ground plan of a lost trait could persist due to pleiotropic effects on other traits and (ii) that gene co-expression network architecture could constrain individual gene expression. Our testing ground has been the Bacillus stick insect species complex, which contains close relatives that are either bisexual or parthenogenetic. After the identification of genes expressed in male reproductive tissues in a bisexual species, we investigated their gene co-expression network structure in two parthenogenetic species. We found that gene co-expression within the male gonads was partially preserved in parthenogens. Furthermore, parthenogens did not show relaxed selection on genes upregulated in male gonads in the bisexual species. As these genes were mostly expressed in female gonads, this preservation could be driven by pleiotropic interactions and an ongoing role in female reproduction. Connectivity within the network also played a key role, with highly connected-and more pleiotropic-genes within male gonad also having a gonad-biased expression in parthenogens. Our findings provide novel insight into the mechanisms which could underlie the production of rare males in parthenogenetic lineages; more generally, they provide an example of the cryptic persistence of a lost trait molecular architecture, driven by gene pleiotropy on other traits and within their co-expression network.

High disparity in repellent gland anatomy across major lineages of stick and leaf insects (Insecta: Phasmatodea)

BACKGROUND

Phasmatodea are well known for their ability to disguise themselves by mimicking twigs, leaves, or bark, and are therefore commonly referred to as stick and leaf insects. In addition to this and other defensive strategies, many phasmatodean species use paired prothoracic repellent glands to release defensive chemicals when disturbed by predators or parasites. These glands are considered as an autapomorphic trait of the Phasmatodea. However, detailed knowledge of the gland anatomy and chemical compounds is scarce and only a few species were studied until now. We investigated the repellent glands for a global sampling of stick and leaf insects that represents all major phasmatodean lineages morphologically via µCT scans and analyzed the anatomical traits in a phylogenetic context.

RESULTS

All twelve investigated species possess prothoracic repellent glands that we classify into four distinct gland types. 1: lobe-like glands, 2: sac-like glands without ejaculatory duct, 3: sac-like glands with ejaculatory duct and 4: tube-like glands. Lobe-like glands are exclusively present in Timema, sac-like glands without ejaculatory duct are only found in Orthomeria, whereas the other two types are distributed across all other taxa (= Neophasmatodea). The relative size differences of these glands vary significantly between species, with some glands not exceeding in length the anterior quarter of the prothorax, and other glands extending to the end of the metathorax.

CONCLUSIONS

We could not detect any strong correlation between aposematic or cryptic coloration of the examined phasmatodeans and gland type or size. We hypothesize that a comparatively small gland was present in the last common ancestor of Phasmatodea and Euphasmatodea, and that the gland volume increased independently in subordinate lineages of the Occidophasmata and Oriophasmata. Alternatively, the stem species of Neophasmatodea already developed large glands that were reduced in size several times independently. In any case, our results indicate a convergent evolution of the gland types, which was probably closely linked to properties of the chemical components and different predator selection pressures. Our study is the first showing the great anatomical variability of repellent glands in stick and leaf insects.

Phasmid species that inhabit colder environments are less likely to have the ability to fly

A vast majority of insects can fly, but some cannot. Flight generally increases how far an individual can travel to access mates, enables the exploitation of additional food resources, and aids in predator avoidance. Despite its functional significance, much remains unknown about the factors that influence the evolution of flight. Here, I use phylogenetic comparative methods to investigate whether average annual temperature or wind speed, two components of the flying environment, is correlated with the evolution of flight using data from 107 species of stick and leaf insects (Insecta: Phasmatodea). I find no association between wind speed and flying ability in this clade. However, I find that colder temperatures are associated with the lack of flying ability. This pattern may be explained by the additional metabolic costs required for insects to fly when it is cold. This finding contradicts previous patterns observed in other insect groups and supports the hypothesis that cold temperatures can influence the evolution of flight.

Plant phylogenetics: The never-ending cycle of evolutionary gains and losses

The Zygnematophyceae is the sister clade to the land plants, but their biology remains mysterious. In a new study, a resolved phylogeny and a scenario for the evolution of multicellularity in that clade are proposed.

Cladomorphus petropolisensis, a New Species of Stick Insect from the Atlantic Forest, Rio de Janeiro, Brazil

Simple Summary

The order Phasmatodea includes the longest Brazilian insects, known by their remarkable morphological and behavioural adaptations for camouflage such as sticks, moss, and leaves; they are predominantly nocturnal and phytophagous insects. Cladomorphus phyllinus Gray, 1835 is one of the most common and best-known stick insect species in Brazil. It feeds mainly on guava leaves, angico, and powder-puff, and reproduces sexually and asexually. Cladomorphus phyllinus presents marked sexual dimorphism in the adult: winged males are significantly smaller in size than females and can reach up to 13 cm, while females can reach 23 cm in length and are apterous. A female specimen collected in the Atlantic Forest in Petrópolis, Rio de Janeiro, Brazil, was compared with a C. phyllinus specimen, identified according to published literature. The differences between the two specimens related to the general size and several morphological characters were observed. In order to add evidence that the recently collected specimen belonged to a species distinct from C. phyllinus, part of the cytochrome oxidase I (COI) gene was sequenced and analysed. The comparative analysis of the COI sequences from the two specimens revealed significant differences that, together with the morphological characters and recorded sympatry of the two specimens, support the existence of a new species, which is described here as Cladomorphus petropolisensis.

Abstract

Cladomorphus petropolisensis sp. nov., a new species of stick insect from Petrópolis, Rio de Janeiro, Brazil, is herein described and compared to the other sympatric species, C. phyllinus Gray, 1835 (Phasmatidae, Cladomorphinae). The description of the new species is supported by morphological and molecular evidence. Kimura-2-parameter (K2P) intraspecific COI divergences among the holotype of C. petropolisensis sp. nov. and C. phyllinus individuals ranged from 2.9% to 4.4%, which are suggestive of distinct species, especially when considering that all Cladomorphus individuals studied were collected in the Petrópolis municipality. The new species can be distinguished from C. phyllinus Gray, 1835 by several characteristics: smaller size, the presence of two spines on the hind femora, the relative longer length of the ovipositor, and spiny tegument, especially in the mesonotum, sculpturing of the operculum of the egg.

Taxonomic revision of the Australian stick insect genus Candovia (Phasmida: Necrosciinae): insight from molecular systematics and species-delimitation approaches

The Phasmida genus Candovia comprises nine traditionally recognized species, all endemic to Australia. In this study, Candovia diversity is explored through molecular species-delimitation analyses using the COIFol gene fragment and phylogenetic inferences leveraging seven additional mitochondrial and nuclear loci. Molecular results were integrated with morphological observations, leading us to confirm the already described species and to the delineation of several new taxa and of the new genus Paracandovia. New Candovia species from various parts of Queensland and New South Wales are described and illustrated (C. alata sp. nov., C. byfieldensis sp. nov., C. dalgleishae sp. nov., C. eungellensis sp. nov., C. karasi sp. nov., C. koensi sp. nov. andC. wollumbinensis sp. nov.). New combinations are proposed and species removed from synonymy with the erection of the new genus Paracandovia (P. cercata stat. rev., comb. nov., P. longipes stat. rev., comb. nov., P. pallida comb. nov., P. peridromes comb. nov., P. tenera stat. rev., comb. nov.). Phylogenetic analyses suggest that the egg capitulum may have independently evolved multiple times throughout the evolutionary history of these insects. Furthermore, two newly described species represent the first taxa with fully developed wings in this previously considered apterous clade.