Esterase isozyme patterns in developing embryos of Brachydanio rerio (zebra danio), Brachydanio abolineatus (pearl danio), and their hybrids

Asymmetric viability in reciprocal crosses of zebrafish Danio rerio and pearl danio Danio albolineatus

Interspecies hybrids have long been studied to further understanding of speciation. Reciprocal crosses sometimes have asymmetric viability, a phenomenon termed 'Darwin's corollary to Haldane's rule'. It has been proposed that this asymmetry is caused by Dobzhansky-Muller incompatibilities between nuclear genes and cytoplasmic factors (e.g., maternal transcripts, mitochondrial genome). The molecular basis of this hypothesis has received little empirical investigation, presumably due to the lack of an appropriate model system. We report a case of extreme asymmetry in viability between reciprocal hybrids of zebrafish Danio rerio and pearl danio Danio albolineatus. Hybrids from D. rerio females × D. albolineatus males (n = 4 crosses) were viable, with 83.2 ± 9.6% surviving from fertilization to 5 days post-fertilization (dpf) and 80.1 ± 14.4% surviving from 5 to 21 dpf. Hybrids from D. albolineatus females × D. rerio males (n = 6 crosses) were inviable after embryonic development. These hybrids developed pericardial oedema at 1 dpf and only 37.2 ± 18.0% survived from fertilization to 5 dpf. Of the 595 larvae alive at 5 dpf, only one juvenile with stunted growth survived to 21 dpf. We propose that given the resources available for the D. rerio model system and the strong asymmetry in viability between reciprocal crosses, these hybrids will allow investigation of the molecular basis for Darwin's corollary to Haldane's rule.

Diisopropylfluorophosphate inhibits acetylcholinesterase activity and disrupts somitogenesis in the zebrafish

Acetylcholinesterase (AChE) activity, localized histochemically, appeared in the nuclei of presumptive somitic mesodermal cells prior to the onset of somitogenesis. AChE activity appeared in a rostro-caudal sequence, in cells located the equivalent of five somite lengths caudal to the last formed somite. To investigate whether AChE activity was required for somitogenesis, several inhibitors of AChE activity were tested for their ability to block somitogenesis. Diisopropylfluorophosphate (DFP), a broad spectrum inhibitor of serine proteases and related enzymes, was the only AChE inhibitor tested that disrupted somitogenesis. Gastrulae at 50% epiboly exposed continuously to DFP at concentrations between 40 microM and 90 microM completed epiboly, but exhibited a dose-dependent decrease in the number of somites formed, and a parallel decrease in the caudal extent of somite innervation, by 24 hours post-fertilization (h). Fifteen somite (15h) embryos exposed to DFP at the ED50 of 70 microM for 3 hours, followed by recovery to 24h, developed abnormal somites. Approximately five normal somites formed after drug treatment before the first abnormal somite formed. The abnormal somites corresponded in location to that area of the presumptive somitic mesoderm that would have initiated AChE activity while the DFP was present. While exposed to 70 microM DFP, presumptive somites formed and motoneurons extended processes that had initiated AChE activity at the time of treatment with DFP, although at a slower than normal rate. However, embryos exposed to 1 mM DFP for 30 minutes at both the 5 and 15 somite stages, followed by recovery to 24h, developed the normal number of somites but were reduced in the caudal extent of somite innervation, and occasionally developed abnormal primary motoneurons. As with the abnormal somites, the abnormal motoneurons would have initiated AChE activity while the DFP was present. Presumptive somitic mesoderm unable to initiate AChE activity due to inhibition by DFP developed abnormally. While the effects of DFP are not limited to inhibiting AChE, the data support the "clock and wavefront" model proposed for somite formation, and support the hypothesis that AChE activity has a role in somitogenesis in zebrafish.

Physicochemical characterization and tissue distribution of multiple molecular forms of fish (Trachurus trachurus) esterases

1. Tissue-specific electrophoretic patterns of multiple molecular forms of esterases were observed in fish Trachurus trachurus. They were composed of three (in intestine) to nine (in liver) bands characterized mainly as carboxylesterases and acetylesterases. 2. Two major esterase activity bands of pI 4.60 and 4.77 were accompanied by minor ones of higher pH in tissue extracts. 3. Under optimum assay conditions, liver was the richest source of esterase activity followed by intestine, stomach, brain, red muscles, heart, white muscles and gills. 4. Esterases of 70,000 and 420,000 mol. wt were resolved by gel-filtration in liver, intestine and brain. Low size esterases prevailed in liver while the opposite was the case in brain.

Developmental expression of glucose and triose phosphate isomerase genes in teleost fishes (brachydanio)

The ontogeny of glucosephosphate isomerase (GPI; E.C. 5.3.1.9) and triosephosphate isomerase (TPI; E.C. 5.3.1.1) isozymes in Brachydanio rerio, (zebra danio), B. albolineatus (pearl danio) and hybrids formed by their reciprocal crosses were examined by electrophoretic and spectrophotometric methods. The two species showed virtually identical adult, tissue-specific distributions and developmental progressions for both GPI and TPI isozymes. The Gpi-A and Tpi-B loci were expressed in all tissues studied. The Gpi-B locus was predominantly expressed in skeletal muscle and the Tpi-A locus in eye, brain and ovary. The GPI-A2, TPI-A2, TPI-AB and TPI-B2 isozymes were continuously expressed from fertilization through 5 days postfertilization. The GPI-B2 isozyme initially appeared at 35 hours postfertilization, temporally correlating with the early differentiation of trunk somite myoblasts. Spectrophotometric analysis of total GPI showed fluctuations in activity through 48 hours, followed by a steady increase through 4 days postfertilization. Total TPI showed increased activity by the end of gastrulation and a sharp rise in activity beginning at 25 hours postfertilization. Embryos of reciprocal hybrids between B. rerio and B. albolineatus showed that isozymes derived from the activation of the paternal Tpi-B, Gpi-A and Tpi-A alleles were initially detected at 20, 25, and 48 hours following fertilization, respectively. The maternal and paternal isozymes of the Gpi-B locus were synchronously expressed at 35 hours postfertilization. The pattern of GPI-B2 expression in hybrids followed a pattern similar to that in intraspecific crosses. Exposure of B. rerio embryos to pulses of actinomycin D at various stages prior to 35 hours postfertilization showed that messenger RNA encoding the GPI-B2 isozyme was first transcribed over a 14-hour interval initiated during mid-gastrulation. Translation of the GPI-B2 message occurred during a 13-hour period immediately preceding the embryonic expression of the GPI-B2 isozyme. Our studies with Brachydanio indicate that several gene loci are activated at gastrulation and transcribe messenger RNA molecules coding for histospecific isozymes associated with later stages of organogenesis.