Assessing Teratogenicity from the Clustering of Abnormal Phenotypes in Individual Zebrafish Larvae

In previous publications, we described the population incidence of abnormalities in zebrafish larvae exposed to toxicants. Here, we examine the phenomenon of clustering or co-occurrence of abnormalities in individual larva. Our aim is to see how this clustering can be used to assess the specificity and severity of teratogenic effect. A total of 11,214 surviving larvae, exposed continuously from 1 day postfertilization (dpf) to one of 60 toxicants, were scored at 5 dpf for the presence of eight different abnormal phenotypes. These were as follows: pericardial edema, yolk sac edema, dispersed melanocytes, bent tail, bent trunk, hypoplasia of Meckel's cartilage, hypoplasia of branchial arches, and uninflated swim bladder. For 43/60 compounds tested, there was a concentration-dependent increase in the severity score (number of different abnormalities per larva). Statistical analysis showed that abnormalities tended to cluster (i.e., to occur in the same larva) more often than expected by chance alone. Yolk sac edema and dispersed melanocytes show a relatively strong association with one another and were typically the first abnormalities to appear in single larvae as the concentration of compound was increased. By contrast, hypoplastic branchial arches and hypoplastic Meckel's cartilage were only frequently observed in the most severely affected larvae. We developed a metric of teratogenicity (TC_3/8), which represents the concentration of a compound that produces, on average, 3/8 abnormalities per larva. On this basis, the most teratogenic compounds tested here are amitriptyline, chlorpromazine hydrochloride, and sodium dodecyl sulfate; the least teratogenic is ethanol. We find a strong correlation between TC_3/8 and LC₅₀ of the 43 compounds that showed teratogenic effects. When we examined the ratio of TC_3/8 to LC₅₀, benserazide hydrochloride, copper (II) nitrate trihydrate, and nicotine had the highest specific teratogenicity, while aconitine, hesperidin, and ouabain octahydrate had the lowest. We conclude that analyzing the clustering of abnormalities per larva can provide an enriched teratogenic dataset compared with simple measurement of the population frequency of abnormalities.