Direct development in African squeaker frogs (Anura: Arthroleptidae: Arthroleptis) reveals a mosaic of derived and plesiomorphic characters

Parallel evolution of direct development in frogs ‐ Skin and thyroid gland development in African Squeaker Frogs (Anura: Arthroleptidae: Arthroleptis)

“…19,23,27,33,[67][68][69][70] There are many similarities between E. coqui and Arthroleptis regarding metamorphic repatterning processes that correlate with histological thyroid gland maturation, such as tail regression, remodeling of the "larval" into the adult hyobranchial apparatus, and cranial muscle repatterning. 6,19,27,33,71 Most of these processes start slightly earlier in Arthroleptis, which correlates with an earlier appearance of the thyroid glands compared to E. coqui (Jennings & Hanken, 1998; this study). Assuming that histologically inferred thyroid gland maturity roughly correlates with its activity and the production of TH, Ischnocnema henselii might be another example where some features of skin development might have lost their dependency on TH.…”

“…Embryos of Arthroleptis wahlbergii Smith, 1849, collected in South Africa, and A. xenodactyloides Hewitt, 1933, collected in Tanzania, were euthanized using 0.1% tricaine methanesulfonate (MS222, Fluka), fixed in 4% phosphate buffered formalin and stored in 70% ethanol. 6 Staging of embryos follows Townsend and Stewart (1985); TS stages hereafter. One tadpole of Cardioglossa manengouba Blackburn, 2008 was available for investigation.…”

“…1 A potential case of parallel evolution is the repeated, independent origin of direct development within frogs (Anura) and other amphibians. [4][5][6][7] Direct development is characterized by the loss of an aquatic larval phase 8 and may have evolved in response to environmental conditions that restricted the availability of suitable habitats for aquatic development. [9][10][11][12] In frogs, direct development evolved several times independently (Figure 1(A)), making them a suitable model system to unravel developmental constraints that underlay patterns of parallel evolution.…”

“…18,19 However, morphological studies on the embryonic development of direct-developing frogs are limited to only a few species. 6,13,[20][21][22] The only species in which direct development has been investigated in greater detail is the Puerto Rican coqui, Eleutherodactylus coqui Thomas, 1966. 19,23-34 Although direct-developing frogs have departed profoundly from their ancestral ontogeny 6,19,35 , studies in E. coqui have shown that several developmental events are still under the control of TH 23,36 and that it undergoes a cryptic metamorphosis.…”

“…6,13,[20][21][22] The only species in which direct development has been investigated in greater detail is the Puerto Rican coqui, Eleutherodactylus coqui Thomas, 1966. 19,23-34 Although direct-developing frogs have departed profoundly from their ancestral ontogeny 6,19,35 , studies in E. coqui have shown that several developmental events are still under the control of TH 23,36 and that it undergoes a cryptic metamorphosis. 23,37 Ontogenetic repatterning seen in direct-developing frogs is thought to be the result of changes in the expression of TH, which leads to changes in timing of TH-dependent events during development ("heterochronic shift" hypothesis).…”