Abstract
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.
Highlights
We addressed this question with a simple approach and used a CAPA antiserum raised against a cockroach CAPA peptide to screen the ventral nerve cord (VNC) of the fire ant, Solenopsis invicta, for CAPA immunoreactive cells
A comparison of capa gene transcription in different parts of the CNS of adult females indicated that the long transcript is more abundant in brain and subesophageal ganglion (SEG) whereas the short form is slightly more abundant in abdominal ganglia (Fig. 2B)
Expressed in at least two ventral neurosecretory cells of abdominal ganglia (Vaneurons in Drosophila melanogaster [12,13], VL-cells in P. americana [14], NS-M4 neurons in Manduca sexta [15,16]), these peptides are transported into neurohemal release sites that are usually developed as distinct abdominal perisympathetic organs (PSOs) [17]
Summary
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 [2] We addressed this question with a simple approach and used a CAPA antiserum raised against a cockroach CAPA peptide to screen the ventral nerve cord (VNC) of the fire ant, Solenopsis invicta, for CAPA immunoreactive (ir) cells. The distribution of capa neurons in the VNC of S. invicta is compared with that in another ant, Atta sexdens, and the honey bee, Apis mellifera
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