The fate of the onychophoran antenna

Abstract

Recent gene expression data suggest that the region on which the onychophoran antenna is situated corresponds to the anteriormost, apparently appendage-less region of the arthropod head. The fate of the onychophoran antenna (or any appendage-like precursor), also called the primary antenna, has been discussed intensively, and there are conflicting suggestions that this anteriormost non-segmental appendage gave rise either to the arthropod labrum or, alternatively, to the so-called frontal filaments found in certain crustaceans. Our data on early axogenesis in anostracan crustaceans show that even in the earliest embryos, before the antennula and antennal nerves are developed, the circumoral anlagen of the brain display very prominent nerves which run into the frontal filament organ (also known as the cavity receptor organ). This situation resembles the development of the antennal nerves in onychophorans, which leads us to conclude that the frontal filaments are indeed homologous to the primary antenna. Frontal filaments also appear to be more common in crustaceans than previously thought, removing the need for a complicated scenario of transformation from a primary antenna into the labrum.

Nervous system development in the fairy shrimpBranchinellasp. (Crustacea: Branchiopoda: Anostraca): Insights into the development and evolution of the branchiopod brain and its sensory organs

“…It also provides a detailed survey of the formation of the postnaupliar and peripheral nervous system including the posterior pioneer neurons. Furthermore, it adds larval and postlarval stages to published embryonic data on Anostraca (Frase and Richter, ) and completes our developmental investigations into Branchiopoda with results obtained using immunohistochemical methods.…”

“…To make it possible to study the embryos, some of the cysts were subjected to the process described in Frase and Richter (). Since only a limited number of cysts were handled in this way, we were only able to study a few older embryonic stages.…”

Origins and early evolution of arthropods

“…Eriksson et al . () postulated that the protocerebral antennae of onychophorans and the labrum of arthropods are homologous on the basis of Hox gene alignment, whereas Frase and Richter () instead homologized onychophoran antennae with the frontal filaments of crustaceans.…”

Comparative analysis of gene expression patterns in the arthropod labrum and the onychophoran frontal appendages, and its implications for the arthropod head problem

“…Nevertheless, some authors doubt its homology with the other limbs and instead suggest that the labrum may have evolved independently, and that the apparent genetic similarities are the result of convergent evolution rather than homology [3, 11–13, 16, 17]. …”

“…Morphological studies and gene expression analysis of conserved Hox gene patterning in extant arthropods and onychophorans revealed that the onychophoran frontal appendages are not homologous with the mandibulate antennae (despite their morphological and functional similarity) but instead represent more anterior appendages [22–26]. Therefore, the idea has been put forward that the labrum is homologous with the frontal appendages of extant onychophorans and stem-group lobopodians and the great appendages of stem-group arthropods (e.g., [27–30]), but see [3, 16, 17] for another homology hypothesis.…”