Date Published: April 9, 2014
Publisher: Public Library of Science
Author(s): Man-Yeon Choi, Rene Köhler, Robert K. Vander Meer, Susanne Neupert, Reinhard Predel, Wolfgang Blenau.
Recent genome analyses suggested the absence of a number of neuropeptide genes in ants. One of the apparently missing genes was the capa gene. Capa gene expression in insects is typically associated with the neuroendocrine system of abdominal ganglia; mature CAPA peptides are known to regulate diuresis and visceral muscle contraction. The apparent absence of the capa gene raised questions about possible compensation of these functions. In this study, we re-examined this controversial issue and searched for a potentially unrecognized capa gene in the fire ant, Solenopsis invicta. We employed a combination of data mining and a traditional PCR-based strategy using degenerate primers designed from conserved amino acid sequences of insect capa genes. Our findings demonstrate that ants possess and express a capa gene. As shown by MALDI-TOF mass spectrometry, processed products of the S. invicta capa gene include three CAPA periviscerokinins and low amounts of a pyrokinin which does not have the C-terminal WFGPRLa motif typical of CAPA pyrokinins in other insects. The capa gene was found with two alternative transcripts in the CNS. Within the ventral nerve cord, two capa neurons were immunostained in abdominal neuromeres 2–5, respectively, and projected into ventrally located abdominal perisympathetic organs (PSOs), which are the major hormone release sites of abdominal ganglia. The ventral location of these PSOs is a characteristic feature and was also found in another ant, Atta sexdens.
Although higher insect taxa (“nsect order” diverged more than 200 mya, general features of the hexapod neuroendocrine system, including cell location, hormone release sites, and sequences of the peptide hormones are strongly conserved. This conservation reflects the functional significance of these peptide messenger molecules. Therefore, it was unexpected that recent genome analyses failed to identify several neuropeptide genes in a number of insects. For the key substances of the segmentally arranged perisympathetic organs (PSOs), which store products of the extended fmrf gene in the thorax and products of the capa gene in the abdomen, this situation was first described from the parasitic wasp Nasonia vitripennis (Hymenoptera; ). Here, the absence of otherwise highly conserved peptide hormones could be attributed to the miniaturization of the nervous system or parasitic lifestyle but later it was described that ant genomes do not possess fmrf and capa genes as well .
CAPA-peptides are typically synthesized from the neuroendocrine system of the abdominal VNC in insects . Expressed in at least two ventral neurosecretory cells of abdominal ganglia (Va-neurons in Drosophila melanogaster, , VL-cells in P. americana, NS-M4 neurons in Manduca sexta, ), these peptides are transported into neurohemal release sites that are usually developed as distinct abdominal PSOs . The mature neuropeptide species (CAPA-PVKs and CAPA-PK) processed from the CAPA precursor were structurally first identified in the cockroach P. americana, . Current available genome data, sequence similarities of processed peptides and the corresponding receptors suggest a common ancestor of capa and pk genes. Possibly within the Pancrustacea lineage, a gene duplication of this predecessor occurred which, in turn, might have a common ancestor with neuromedin U of vertebrates . In insects, the pk gene is known by different names (e. g. pban of Lepidoptera, hugin of Drosophila) but is always expressed in neurosecretory neurons of the SEG , – which is different from the major expression site of the capa gene. CAPA-PVKs and CAPA-PK (C-terminal WFGPRLamide) both have specific receptors , .