Haploidy-linked disruption of tissue organization in zebrafish larval eyes

In vertebrate species, somatic haploidy generally causes embryonic or larval lethality with drastic morphological defects. However, the detrimental effects of haploidy at the cellular or tissue level remain largely unknown. In this study, we described cellular and tissue defects associated with haploidy in zebrafish larval eyes as a tissue model system. The haploid larval retina showed overall disorganized tissue layer structures. Photoreceptor cells, which usually align in the outermost tissue layer, were reduced in number and severely displaced, with drastic retardation of their connecting cilia and outer segments specifically in haploids. Ganglion cells in the inner retinal layer were also reduced in abundance and abnormally organized in haploids. These multifaceted cellular and tissue defects indicate that haploid eyes suffer severe visual functional defects, in addition to well-recognized size reductions. Our findings provide insight into diverse cellular principles that may limit the developmental capability of vertebrates in the haploid state.