Zebrafish Rnf111 is encoded by multiple transcripts and is required for epiboly progression and prechordal plate development.

Arkadia (also known as RING finger 111) encodes a nuclear E3 ubiquitin ligase that targets intracellular effectors and modulators of TGFß/Nodal-related signaling for polyubiquitination and proteasome-dependent degradation. In the mouse, loss of Arkadia results in early embryonic lethality, with defects attributed to compromised Nodal signaling. Here, we report the isolation of zebrafish arkadia/rnf111, which is represented by 5 transcript variants. arkadia/rnf111 is broadly expressed during the blastula and gastrula stages, with eventual enrichment in the anterior mesendoderm, including the prechordal plate. Morpholino knockdown experiments reveal an unexpected role for Arkadia/Rnf111 in both early blastula organization and epiboly progression. Using a splice junction morpholino, we present additional evidence that arkadia/rnf111 transcript variants containing a 3' alternative exon are specifically required for epiboly progression in the late gastrula. This result suggests that arkadia/rnf111 transcript variants encode functionally relevant protein isoforms that provide additional intracellular flexibility and regulation to the Nodal signaling pathway.

Six1a is required for the onset of fast muscle differentiation in zebrafish.

Vertebrate skeletal muscles arise from two major types of precursor cell populations which differentiate into slow and fast fibers. Six1 homeodomain transcription factor was implicated in myogenesis in mammals, but its role in the development of different types of muscle precursors remained unclear. In zebrafish, there are two close homologs of Six1: six1a (known earlier as six1) and six1b identified in this study. Here we studied the role of six1a whose expression is initiated in the fast muscle precursor region of the forming somite. In the six1a loss-of-function conditions, initiation of myog expression was compromised in fast muscle precursors whereas myod expression appeared unaffected suggestive of six1a requirement for fast muscle differentiation. Expression of myog recovered soon, but differentiation of fast muscle proceeded abnormally. Exclusion of muscle-specific transcripts, myhz1 and tpma, from the dorsal and posterior part of somites demonstrated early abnormalities in fast muscle formation. U-shaped somites, reduced birefringence, and abnormal cell morphology were observed in morphant fast muscle upon terminal differentiation. In contrast, differentiation of slow fibers appeared largely unaffected. We conclude that Six1a plays an essential role at the onset of fast muscle differentiation.

Expression of zebrafish six1 during sensory organ development and myogenesis.

Drosophila sine oculis homologous genes in vertebrates are homeobox-containing transcription factors functioning within the Pax-Six-Eya-Dach regulatory network during development. In this study, we describe the cloning and expression of a zebrafish homolog of sine oculis, six1. The reverse transcription-polymerase chain reaction demonstrated accumulation of six1 transcripts at mid-gastrula, and in situ hybridization showed their subsequent expression in the cranial placode and later in the olfactory, otic, and lateral line placodes, inner ear, and neuromasts. In addition, six1 is expressed in the pituitary, branchial arches, somites, pectoral fin, ventral abdomen muscle, and the cranial muscles of the eye and lower jaw. An increase of six1 expression was observed in the lateral line, muscles, and inner ear of the mind bomb mutant, illustrating a regulatory effect of the Notch pathway on expression of Six genes.

Two Pax genes, eye gone and eyeless, act cooperatively in promoting Drosophila eye development.

We report the identification of a Drosophila Pax gene, eye gone (eyg), which is required for eye development. Loss-of-function eyg mutations cause reduction or absence of the eye. Similar to the Pax6 eyeless (ey) gene, ectopic expression of eyg induces extra eye formation, but at sites different from those induced by ey. Several lines of evidence suggest that eyg and ey act cooperatively: (1) eyg expression is not regulated by ey, nor does it regulate ey expression, (2) eyg-induced ectopic morphogenetic furrow formation does not require ey, nor does ey-induced ectopic eye production require eyg, (3) eyg and ey can partially substitute for the function of the other, and (4) coexpression of eyg and ey has a synergistic enhancement of ectopic eye formation. Our results also show that eyg has two major functions: to promote cell proliferation in the eye disc and to promote eye development through suppression of wg transcription.