Pax6 in Collembola: Adaptive Evolution of Eye Regression

It takes Two: Discovery of Spider Pax2 Duplicates Indicates Prominent Role in Chelicerate Central Nervous System, Eye, as Well as External Sense Organ Precursor Formation and Diversification After Neo- and Subfunctionalization

Paired box genes are conserved across animals and encode transcription factors playing key roles in development, especially neurogenesis. Pax6 is a chief example for functional conservation required for eye development in most bilaterian lineages except chelicerates. Pax6 is ancestrally linked and was shown to have interchangeable functions with Pax2. Drosophila melanogaster Pax2 plays an important role in the development of sensory hairs across the whole body. In addition, it is required for the differentiation of compound eyes, making it a prime candidate to study the genetic basis of arthropod sense organ development and diversification, as well as the role of Pax genes in eye development. Interestingly, in previous studies identification of chelicerate Pax2 was either neglected or failed. Here we report the expression of two Pax2 orthologs in the common house spider Parasteatoda tepidariorum, a model organism for chelicerate development. The two Pax2 orthologs most likely arose as a consequence of a whole genome duplication in the last common ancestor of spiders and scorpions. Pax2.1 is expressed in the peripheral nervous system, including developing lateral eyes and external sensilla, as well as the ventral neuroectoderm of P. tepidariorum embryos. This not only hints at a conserved dual role of Pax2/5/8 orthologs in arthropod sense organ development but suggests that in chelicerates, Pax2 could have acquired the role usually played by Pax6. For the other paralog, Pt-Pax2.2, expression was detected in the brain, but not in the lateral eyes and the expression pattern associated with sensory hairs differs in timing, pattern, and strength. To achieve a broader phylogenetic sampling, we also studied the expression of both Pax2 genes in the haplogyne cellar spider Pholcus phalangioides. We found that the expression difference between paralogs is even more extreme in this species, since Pp-Pax2.2 shows an interesting expression pattern in the ventral neuroectoderm while the expression in the prosomal appendages is strictly mesodermal. This expression divergence indicates both sub- and neofunctionalization after Pax2 duplication in spiders and thus presents an opportunity to study the evolution of functional divergence after gene duplication and its impact on sense organ diversification.

Analysis of corneal morphologic and pathologic changes in early-stage congenital aniridic keratopathy

AIM

To determine typical corneal changes of congenital aniridic keratopathy (CAK) using corneal topography and confocal systems, and to identify characteristics that might assist in early diagnosis.

METHODS

Patients with CAK and healthy control subjects underwent detailed ophthalmic examinations including axial length, corneal thickness, tear film condition, corneal topography, and laser-scanning in vivo confocal microscopy (IVCM).

RESULTS

In early stage aniridic keratopathy, Schirmer I test (SIT), break-up time (BUT), mean keratometry (mean K) and simulated keratometry (sim K) were reduced relative to controls (P<0.05), while simulation of corneal astigmatism (sim A) and corneal thickness were increased (P<0.05). In addition, significantly more eyes exhibited flat cornea compared with the control group. Inflammatory dendritic cells were present in the aniridic epithelium, with significantly increased density relative to controls (P<0.05). Palisade ridge-like features and abnormal cell morphology were observed in six out of sixteen CAK cases. In central cornea area, the aniridic corneas had the increased subbasal nerve density.

CONCLUSION

These changes in corneal morphology in borderline situations can be useful to confirm the diagnosis of CAK.