The fate of the onychophoran antenna
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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.
KeywordsLabrum Primary antenna Secondary antenna Frontal filament Organ of bellonci
We would like to thank Martin Fritsch for the detailed discussions on the subject of frontal filament organs and the development of branchiopod nervous systems. We are grateful to Martin Schwentner who checked the species identity of A. franciscana by DNA barcoding. Lucy Cathrow improved the English which is gratefully acknowledged. Two anonymous reviewers provided helpful suggestions, which are also acknowledged.
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