Development of the Pre-Gnathal Segments of the Insect Head Indicates They Are Not Serial Homologues of Trunk Segments

The three anterior-most segments in arthropods contain the ganglia that make up the arthropod brain. These segments, the pre-gnathal segments, are known to exhibit many developmental differences to other segments, believed to reflect their divergent morphology. We have analyzed the expression and function of the genes involved in the segment-polarity network in the pre-gnathal segments compared with the trunk segments in the hemimetabolous insect Oncopeltus fasciatus. We show that there are fundamental differences in the way the pre-gnathal segments are generated and patterned, relative to all other segments, and that these differences are general to all arthropods. We argue that given these differences, the pre-gnathal segments should not be considered serially homologous to trunk segments. This realization has important implications for our understanding of the evolution of the arthropod head. We suggest a novel scenario for arthropod head evolution that posits duplication of an ancestral single-segmented head into three descendent segments. This scenario is consistent with what we know of head evolution from the fossil record, and helps reconcile some of the debates about early arthropod evolution.

Evolution and Radiation of Crustacea

The evolution of the Crustacea following their origins in the Cambrian is outlined, with an overview of their paleontological history and global distributions into modern times. Major recent developments in arthropod evolution include recognition that Hexapoda is nested within Crustacea. Perspectives also changed during the last decades of the 20th century on the form of the crustacean ancestor, from being a long-bodied, serially homonomous form (like a remipede or cephalocarid) to a short-bodied, possibly ostracod-like form similar to Cambrian stem and crown group fossil forms. These changes have come through a shift to formal methods of phylogenetic analysis combined with the much larger volume of both morphological and molecular data now available. The most extensive current phylogenies typically recover the short-bodied Oligostraca (containing ostracods and a few minor groups) as basal crustaceans; Malacostraca and Maxillopoda are high in the tree; and Cephalocarida and Remipedia are derived forms as sister to Branchiopoda and Hexapoda, respectively. Each of these major groups can be understood through variations in tagmatization (differentiation of body segments into regions). The early crustacean fossil record (especially the Ordovician) is dominated by ostracods. Malacostracans, although having Cambrian origins, did not significantly radiate until the Mesozoic. Eumalacostraca continued to actively radiate in the Cenozoic and are now the most ubiquitous and morphologically disparate crustaceans. The processes driving crustacean evolution remain to be fully evaluated. Contingency and external factors are undoubtedly important, but most deep lineages of the Crustacea show pervasive macroevolutionary trends toward increasing tagmatization. These trends are apparently driven, meaning the formation of new body plans is not merely a contingent outcome—intrinsic factors may contribute to increasing tagmatization. Further data are required from ontogeny and developmental genetics, paleontology, and phylogenetics in order to better understand how crustaceans have evolved.

Patterns of Larval Development

In this chapter, we explore the different patterns of development following the hatching of the crustacean larvae. For many groups of crustaceans, the free-living, postembryonic, and prejuvenile phase is by far the most important part of their life cycle, providing the link between different life modes in successive phases (e.g., between a sessile adult life and the need for long-range planktonic dispersal). Among the aspects covered, we discuss the specific criteria for what a “larva” is, including the necessity for defining specific larval traits that are lacking in other phases of the life cycle. We examine the typical anamorphic and hemianamorphic developmental patterns based on larval examples from a wide selection of groups from Decapoda to Copepoda, Thecostraca to Branchiopoda. In these groups, we examine the most common larval development patterns (including intraspecific variability) of, for example, the zoea, furcilia, copepodite, nauplius, and cypris larvae. We also expand on the importance of the molting cycle as the main driver in larval ontogeny and evolution. Finally, we discuss some of the more general trends of crustacean larval development in light of the general patterns and latest knowledge on tetraconate and arthropod evolution.

Honey bee parasitic mite contains the sensory organ expressing ionotropic receptors with conserved functions

Honey bee parasitic mites (Tropilaelaps mercedesae and Varroa destructor) detect temperature, humidity, and odor but the underlying sensory mechanisms are poorly understood. To uncover how T. mercedesae responds to environmental stimuli inside a hive, we identified the sensilla-rich sensory organ on the foreleg tarsus. The organ contained four types of sensilla, which may respond to different stimuli based on their morphology. We found the forelegs were enriched with mRNAs encoding sensory proteins such as ionotropic receptors (IRs) and gustatory receptors (GRs), as well as proteins involved in ciliary transport. We also found that T. mercedesae and Drosophila melanogaster IR25a and IR93a are functionally equivalent. These results demonstrate that the structures and physiological functions of ancient IRs have been conserved during arthropod evolution. Our study provides insight into the sensory mechanisms of honey bee parasitic mites, as well as potential targets for methods to control the most serious honey bee pest.

Gene Content Evolution in the Arthropods

BackgroundArthropods comprise the largest and most diverse phylum on Earth and play vital roles in nearly every ecosystem. Their diversity stems in part from variations on a conserved body plan, resulting from and recorded in adaptive changes in the genome. Dissection of the genomic record of sequence change enables broad questions regarding genome evolution to be addressed, even across hyper-diverse taxa within arthropods.ResultsUsing 76 whole genome sequences representing 21 orders spanning more than 500 million years of arthropod evolution, we document changes in gene and protein domain content and provide temporal and phylogenetic context for interpreting these innovations. We identify many novel gene families that arose early in the evolution of arthropods and during the diversification of insects into modern orders. We reveal unexpected variation in patterns of DNA methylation across arthropods and examples of gene family and protein domain evolution coincident with the appearance of notable phenotypic and physiological adaptations such as flight, metamorphosis, sociality and chemoperception.ConclusionsThese analyses demonstrate how large-scale comparative genomics can provide broad new insights into the genotype to phenotype map and generate testable hypotheses about the evolution of animal diversity.