An evolutionary insight into the hatching strategies of pipefish and seahorse embryos

Abstract

Syngnathiform fishes carry their eggs in a brood structure found in males. The brood structure differs from species to species: seahorses carry eggs within enclosed brood pouch, messmate pipefish carry eggs in the semi-brood pouch, and alligator pipefish carry eggs in the egg compartment on abdomen. These egg protection strategies were established during syngnathiform evolution. In the present study, we compared the hatching mode of protected embryos of three species. Electron microscopic observations revealed that alligator pipefish and messmate pipefish egg envelopes were thicker than those of seahorses, suggesting that the seahorse produces a weaker envelope. Furthermore, molecular genetic analysis revealed that these two pipefishes possessed the egg envelope-digesting enzymes, high choriolytic enzyme (HCE), and low choriolytic enzyme (LCE), as do many euteleosts. In seahorses, however, only HCE gene expression was detected. When searching the entire seahorse genome by high-throughput DNA sequencing, we did not find a functional LCE gene and only a trace of the LCE gene exon was found, confirming that the seahorse LCE gene was pseudogenized during evolution. Finally, we estimated the size and number of hatching gland cells expressing hatching enzyme genes by whole-mount in situ hybridization. The seahorse cells were the smallest of the three species, while they had the greatest number. These results suggest that the isolation of eggs from the external environment by paternal bearing might bring the egg envelope thin, and then, the hatching enzyme genes became pseudogenized. J. Exp. Zool. (Mol. Dev. Evol.) 9999B:XX-XX, 2016. © 2016 Wiley Periodicals, Inc.

Male parental assistance in embryo hatching of barred‐chin blenny Rhabdoblennius nitidus

“…The size and the number of hatching‐gland cells of barred‐chin blenny were analyzed from the RnHCE‐positive cells after in situ hybridizations. The diameter of the cells was about 11.4 ± 0.8 µm ( SD ; n = 3) along the long axis, which was smaller than of other teleosts (15–30 µm; Hiroi et al, ; Kawaguchi, Nakano et al, ; Morrison, Miyake, & Wright, ). Moreover, we found large differences in the numbers of the cells.…”

“…Another example is fishes in which the male becomes pregnant. This notably includes seahorses, where the eggs are fertilized externally, and then the male immediately begins to care for the embryos in a brood pouch located on the ventral side of the tail (Kawaguchi, Nakano et al, ). Whereas these fishes carry the eggs internally, some fishes care for eggs externally within a nest.…”

“…For instance, the LCE gene is a pseudogene in the ovoviviparous black rockfish Sebastes schlegelii and in the pot‐bellied seahorse Hippocampus abdominalis ; and both the HCE and LCE genes are pseudogenes in the ovoviviparous platyfish Xiphophorus maculatus (Kawaguchi, Nakano et al, ; Kawaguchi et al, ; Kawaguchi, Tomita, Sano, & Kaneko, ). Notably, all those species possess extremely thin egg envelopes as compared with the close oviparous relatives, because the eggs are protected from the physical environmental stress while inside the maternal body or the male brood pouch (Kawaguchi, Nakano et al, ; Sano et al, ). The hatching enzyme gene(s) may have mutated into pseudogene(s) and so became fixed via genetic drift, due to a reduced dependence on the hatching enzymes.…”

Early development of the longsnout seahorse Hippocampus reidi (Syngnathidae) within the male brood pouch

“…In seahorses, the only hatching enzyme that seems to cause swelling is high choriolytic enzyme (HCE) in the thin egg envelope, which was detected by Kawaguchi et al (2016). These authors also described a softening of the egg envelope during development and breaking of the envelope with subsequent embryo movement.…”

The evolution and physiology of male pregnancy in syngnathid fishes

“…These results suggest that a closed pouch produces optimal conditions for development, particuarly until the embryos have developed their own osmoregulatory ability via gills and kidneys (Quast & Howe, ; Partridge et al ., ). The chorion surrounding eggs of species with closed brood pouches ( Hippocampus abdominalis ) is thinner than that of species with open pouches ( Corythoichthys haematopterus , Syngnathoides biaculeatus ) (Kawaguchi et al ., ), perhaps because osmoregulation of the closed pouch fluid removes the requirement for a protective chorion, and chorion thinning reduces the barrier to respiratory gas diffusion. In addition, oxygen becomes less soluble at higher salinities, so by reducing pouch salinity, males may enhance oxygen concentration in the pouch (Monteiro et al ., ).…”