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The study of oogenesis and early development of frogs belonging to the family Hemiphractidae provide important comparison to the aquatic development of other frogs, such as Xenopus laevis, because reproduction on land characterizes the Hemiphractidae. In this review, the multinucleated oogenesis of the marsupial frog Flectonotus pygmaeus (Hemiphractidae) is analyzed and interpreted. In addition, the adaptations associated with the incubation of embryos in the pouch of the female marsupial frog Gastrotheca riobambae (Hemiphractidae) and the embryonic development of this frog are summarized. Moreover, G. riobambae gastrulation is compared with the gastrulation modes of Engystomops randi and Engystomops coloradorum (Leptodactylidae); Ceratophrys stolzmanni (Ceratophryidae); Hyalinobatrachium fleischmanni and Espadarana callistomma (Centrolenidae); Ameerega bilinguis, Dendrobates auratus, Epipedobates anthonyi, Epipedobates machalilla, Epipedobates tricolor, and Hyloxalus vertebralis (Dendrobatidae); Eleutherodactylus coqui (Terrarana: Eleutherodactylidae), and X. laevis (Pipidae). The comparison indicated two modes of frog gastrulation. In X. laevis and in frogs with aquatic reproduction, convergent extension begins during gastrulation. In contrast, convergent extension occurs in the post-gastrula of frogs with terrestrial reproduction. These two modes of gastrulation resemble the transitions toward meroblastic cleavage found in ray-finned fishes (Actinopterygii). In spite of this difference, the genes that guide early development seem to be highly conserved in frogs. I conclude that the shift of convergent extension to the post-gastrula accompanied the diversification of frog egg size and terrestrial reproductive modes.
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29305906
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Fig. 1. The reproduction of Gastrotheca riobambae. (A) Female incubating embryos. The pouch occupies the back and the sides of the body and extends to the back of the head. The pouch aperture (arrow) is rostral to the cloaca. (B) Diagrams of the pouch aperture. The borders of the pouch aperture are separated when the pouch is open, and come together at the mid-line when the pouch is closed. (C) Longitudinal section through pouch and dorsal skin. The pouch is continuous with the dorsal skin at the level of the pouch aperture. (D) Section of an embryo within the pouch. A large embryonic chamber separates the embryo from the pouch-bell gill association. (E) The pouch-bell gill association at higher magnification. The fertilization envelope and egg jelly capsule separate the pouch from the bell gills. Image in (B) is modified from del Pino (1983). Micrograph in (C) by Ruth E. Ruiz. Images in (C, D) are reproduced from Schmid et al. (2012). bg: bell gill, c: cloaca, e: embryo, ec: embryonic chamber, j: fertilization envelope and egg jelly, p: pouch, pa: pouch aperture, p-bg: pouch bell gill association, s: skin. Scale bars: 10 mm (A); 2 mm (C); 500 μm (D); 50 μm (E).
Fig. 2. Oogenesis of Flectonotus pygmaeus and Gastrotheca riobambae. Images in (A–E) are light micrographs. Images in (F–G) are transmission electron micrographs. (A–D) Oocytes of Flectonotus pygmaeus. Diagrams on the left summarize the process of F. pygmaeus oogenesis. Nuclei are depicted in blue. (A) Section through two early oocytes with nuclei of uniform size. (B) Section through two oocytes larger than in (A). Peripheral nuclei are large in comparison with centrally located nuclei. (C) Section through a vitellogenic oocyte with three nuclei. (D) Section through a vitellogenic oocyte with the final nucleus located near the oocyte cortex. The numerous nucleoli are clustered in a karyosphere around the condensed chromosomes. Final oocyte size is 3 mm. (E–G) Oocytes of Gastrotheca riobambae at metamorphosis. (E) Section through the ovary. Cysts of oogonia surround a centrally located oocyte. The oocyte contains a single nucleus (F) Section through an ovarian cyst. Each oogonium has a single nucleus with expanded chromosomes. Oogonia were connected with cytoplasmic bridges (not shown). The total number of oogonia in the ovarian cysts of G. riobambae is unknown. (G) A previtellogenic oocyte. The Balbiani body, or mitochondrial cloud, is asymmetrically located in the cytoplasm. Chromosomes are visible in the nucleus. The white area around nuclei in (C, D) is a processing artifact. The section in (A) comes from the same oocytes shown in del Pino (1989). Images in (C, F) are reproduced from Schmid et al. (2012). Bb: Balbiani body, f: follicle envelope, Fp, Flectonotus pygmaeus; Gr, Gastrotheca riobambae; n: nucleus, nu: nucleolus. Scale bars: 20 μm (A); 100 μm (B); 200 μm (C); 40 μm (E). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 3. Embryonic development in the pouch of Gastrotheca riobambae. The diagrams on the left summarize the process of development. Embryos in (B, C, D) were stained for cell borders with silver nitrate. Embryo in (G) was immunostained with an anti-neural antibody. The embryos in (H, I, J) were immunostained with an anti-myosin antibody. The embryonic disk in (G, I) was dissected from the yolky endoderm and it was mounted on slides. (A) View from the animal pole of an 8-cell embryo. Approximate diameter 3 mm. (B) An early gastrula. The leading edge of the mesendoderm invades the blastocoel roof, and the transparent window of the blastocoel roof is small. Approximate diameter 3.5 mm. (C) A late gastrula. The blastopore is visible. (D) The embryonic disk. After blastopore closure, a disk of small cells surrounds the closed blastopore. (E) Sagittal section through the closed blastopore. The cells that involute during gastrulation remain in a large circumblastoporal collar. The archenteron is very small, and the blastocoel is large. (F) Sagittal section of an embryo after archenteron expansion. The embryo rotates and the embryonic disk and archenteron face upwards. (G) The neurula. The embryo has a flat orientation on the embryonic disk. The streams of cranial neural crest surround the head. (H) The branchial arches. The first and second branchial arches enlarge and become thick. The heart is anterior to the head. (I) Heart beat. The heart is located anterior to the head, has the appearance of a U-shaped tube, and it beats in living embryos. Blood circulation had not started. The branchial arches fused to originate the primordia of bell gills. (J) Advanced embryo with full development of the bell gills. In living embryos, the disk-shaped bell gills enveloped the embryo in a vascularized sac. Micrographs in (A, E, F) by Ingrid Alarcón. Micrograph in (H) by Iván M. Moya. Images in (D, J) are reproduced from Elinson and del Pino (2012). Image in (G) is reproduced from Schmid et al. (2012). a: archenteron, aba: anterior branchial arch, b: blastopore, ba: branchial anterior stream of cranial neural crest, bg: bell gill, bgp: bell gill primordium, bl: blastocoel, blr: blastocoel roof, bp: branchial posterior stream of cranial neural crest, cf.: cleavage furrow, cbc: circumblastoporal collar, d: disk, h: heart, hy: hyoid stream of cranial neural crest, m: mandibular stream of cranial neural crest, nt: neural tube, o: otocyst, pba: posterior branchial arch, s: somite, yp: yolk plug.
Fig. 4. Modes of frog gastrulation. Embryos were immunostained with an anti-Brachyury antibody. (A) Gastrulation mode 1, exemplified by the X. laevis gastrula. Notochord elongation begins in the mid- gastrula. (B) Gastrulation mode 2, exemplified by the G. riobambae gastrula. Notochord elongation begins after closure of the blastopore. Image in (A) is reproduced from Moya et al. (2007). Image in (B) is another micrograph of the embryo shown in Moya et al. (2007). n: notochord, yp: yolk plug.
