Developmental sources of conservation and variation in the evolution of the primate eye

Abstract

Conserved developmental programs, such as the order of neurogenesis in the mammalian eye, suggest the presence of useful features for evolutionary stability and variability. The owl monkey, Aotus azarae, has developed a fully nocturnal retina in recent evolution. Description and quantification of cell cycle kinetics show that embryonic cytogenesis is extended in Aotus compared with the diurnal New World monkey Cebus apella. Combined with the conserved mammalian pattern of retinal cell specification, this single change in retinal progenitor cell proliferation can produce the multiple alterations of the nocturnal retina, including coordinated reduction in cone and ganglion cell numbers, increase in rod and rod bipolar numbers, and potentially loss of the fovea.

Fig. 1.

Nocturnal and diurnal New World monkeys. (A) Phylogeny of the primates, in the order of distance from great apes, showing presence of nocturnal (blue) and diurnal species. “Stem anthropoids” begin at Tarsius bancanus, which are nocturnal. All past and present Platyrrhine and Catarrhine monkeys (from Callimico down) are thought to be diurnal, with the exception of the recently nocturnal Aotus (arrow). The nocturnal owl monkey, Aotus azarae (B), and the diurnal capuchin monkey, Cebus apella (C), were the two New World monkeys examined in this study.

Fig. 2.

Nocturnal adaptations of Aotus azarae retinas. (A–C) Total retinal area (A), numbers of cones and rods photoreceptors per retinal area (B), and numbers of retinal ganglion cells per retinal area (C) as measured from adult primate retinas of different species, including Aotus azarae and Cebus apella. (D) Representative pictures of immunopositive dissociated retinal cells stained with indicated antibodies. Stained cells are shown in purple and nuclei are shown in green. (E) The proportion of Chx10- and PKCα-stained cells was determined from three adult Aotus azarae (filled bars) or Cebus Apella (open bars). (F and G) Quantification of the proportion of bipolar cells as determined from Chx10 (F) or PKCα (G) immunostainings in adult retinal sections of both Aotus (filled bars) and Cebus (open bars). The number of Chx10- or PKCα-stained cells was quantified in central and peripheral areas of the retina. Error bars indicate SEM. (H and I) Representative pictures of adult retinal sections stained for either Chx10 (H) or PKCα (I) are shown for both species. ONL, outer nuclear layer; INL, inner nuclear layer; GCL, ganglion cell layer. (Scale bar, 10 μm.)

Fig. 3.

Retinal progenitor cell proliferation in the developing Aotus and Cebus retinas. (A–C) Predictive illustrations showing how the extension of retinal progenitor cells proliferation during development may lead to changes in retinal cell type distribution as observed in diurnal (B) or nocturnal (C) primate retina. (D) Representative pictures of dissociated retinal progenitor cells previously labeled with markers of DNA synthesis. Retinal explants were cultured in the presence of [³H]thymidine or BrdU (10 μM), dissociated, immunostained for BrdU (red), and overlaid with autoradiographic emulsion. (E–G) The kinetics of retinal progenitor cells proliferation was measured by determining the proportion of [³H]thymidine-positive cells in both Aotus azarae (E) and Cebus apella retinas (F and G). Error bars represent the standard deviation. (H) Quantification of the proportion [³H]thymidine/proliferating cells from various developmental stages from both species indicates that the period of cytogenesis is extended in developing retinas of nocturnal species (Aotus) compared with diurnal ones (Cebus). A total of 250 cells was scored for each sample at each time point in duplicate.

Fig. 4.

Expression of cell cycle regulators in developing Aotus and Cebus retinas. (A) Length of G₁, S, and G₂ phases of the cell cycle as measured from various stages of both Aotus (red) and Cebus (blue) developing retinas. (B and C) Real-time RT-PCR analysis showing normalized levels of cyclin D1 (B), and p27^Kip1 (C) mRNA expression from various developmental stages of Aotus (red) and Cebus (blue) retinas. All samples were normalized to GAPDH expression. (D) Representative pictures of dissociated retinal cells immunostained for p27^Kip1. (E) Quantification of the proportion of p27^Kip1-immunopositive cells from various stages of both Aotus (red) and Cebus (blue) developing retinas. The expression of cell cycle regulator mRNA and protein consistently matched the changes in cell proliferation during the development of Aotus retina compared with Cebus.

Fig. 5.

Developmental regulation of eye growth in diurnal vs. nocturnal primates. (A) Representative pictures of Aotus azarae and Cebus apella skulls. (B and C) Predictive models illustrating the specification of eye size during the development of Aotus and Cebus prenatally during retinogenesis or postnatally by emmetropization. (D) Quantification of eye diameter in relation to brain weight (in grams) in Aotus (red) and Cebus (blue). (E) Quantification of eye diameter in relation to maturational age in Aotus (red) and Cebus (blue).