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首页生态因素与青蛙直接发育进化:从蝌蚪到成蛙的演变
生态因素与青蛙直接发育进化:从蝌蚪到成蛙的演变
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更新于2024-08-07
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本文《无蝌蚪的青蛙:两栖类直接发育的演化》由Elizabeth M. Callery、Hung Fang和Richard P. Elinson三位作者共同探讨了两栖动物进化历程中的一个重要现象——直接发育。直接发育是指个体跳过有明显形态差异的幼体阶段,直接发育成成体形态。论文的焦点集中在生态因素如何驱动这种高级生命史策略在蛙类中的演变。 在传统的两栖类发育模式中,幼体(蝌蚪)与成体形态有显著区别,经历了变态过程。然而,像波多黎各树蛙Eleutherodactylus coqui这样的物种展示了直接发育的实例,其幼体发育过程中发生了一系列显著的改变。这些变化包括适应大型卵的发育策略,以及蝌蚪结构的丧失,使得幼体可以直接发育成与成体相近的形态。 令人意外的是,尽管经历了直接发育,Puerto Rican tree frog仍然经历了一种依赖甲状腺激素的形态转变,这一过程发生在孵化之前。这表明,在直接发育的演化过程中,原始的双相发育模式可能经历了重组和优化,使得发育过程更为高效。 文章深入分析了这种直接发育模式的形成机制,可能涉及对环境压力的适应,比如节省能量和时间,避免寄生虫感染等。此外,它还探讨了可能的遗传和生理机制,以及这些改变如何在进化过程中逐渐累积,最终导致了现代两栖类中的直接发育现象。 这篇论文不仅提供了关于生物进化和发育生物学的重要见解,也揭示了自然选择如何塑造动物的生活史策略,使之更适应特定的生态环境。通过研究Puerto Rican tree frog这样的例子,科学家们得以探索生物进化的复杂性,并为理解其他物种的发育模式提供有价值的参考。
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Frogs without polliwogs: Evolution
of anuran direct development
Elizabeth M. Callery,
1
Hung Fang,
2
and Richard P. Elinson
3
*
Summary
Direct development is the assumption of the adult
morphology without progression through an intervening,
morphologically distinct, free-living larval phase. We
discuss the ecological factors contributing to the evolu-
tion of this derived life-history strategy in frogs, and the
developmental modifications that facilitate such an
unusual mode of embryogenesis. Studies on the Puerto
Rican tree frog,
Eleutherodactylus coqui
, have identified
several such modifications, including developmental
adaptations for dealing with increased egg size, and loss
of tadpole structures. Surprisingly, this direct developer
still undergoes a thyroid hormone-dependent metamor-
phosis, which occurs before hatching. We suggest how
the ancestral biphasic developmental pattern may have
been rearranged during the evolution of direct devel-
opment. BioEssays 23:233±241, 2001.
ß 2001 John Wiley & Sons, Inc.
Introduction
The aim of development is the production of a fertile adult.
Many animals achieve this in a biphasic process. Firstly, the
egg develops into a motile, feeding larva. Secondly, at the end
of the larval stage, which is essentially a period of develop-
mental stasis, the organism metamorphoses and adopts the
adult form. Biphasic development is a successful life-history
strategy, occurring throughout the animal kingdom. There
have been many instances, however, where the biphasic life
history has been radically remodelled by deletion of the larval
stage. This mode of ontogeny is termed direct development
and has evolved in several phyla. Direct development has
been studied intensively in urochordates
(1)
and echino-
derms,
(2)
and also occurs in other phyla, such as molluscs.
(3)
Here, we discuss the loss of the tadpole in the anuran amphi-
bians, focusing on an extreme species of direct developer, the
Puerto Rican tree frog, Eleutherodactylus coqui.
Direct development in the Anura
E. coqui is an entirely terrestrial species belonging to a genus
containing approximately 500 species, all of which are direct
developers.
(4)
A clutch of approximately 40 large eggs, each
twenty times the volume of a Xenopus laevis egg, is incubated
by the father for three weeks, at which point tiny froglets
emerge from their jelly capsules.
(5)
Selected developmental
stages are shown in Fig. 1. There is no free-living larval stage,
and many tadpole-specific features have been eliminated from
development, including larval mouthparts, with associated
muscles and cartilages, cement gland, lateral line organs and
a coiled gut.
(6)
How might this mode of development have evolved? The
paucity of the fossil record precludes the conclusive determi-
nation of whether the ancestral amphibian developmental
mode was biphasic or direct. Among extant anurans, biphasic
development is considered primitive, with direct development
arising independently on multiple occasions.
(7)
Based on other
members of the family Leptodactylidae, to which the genus
Eleutherodactylus belongs, direct development in Eleuther-
odactylus is clearly a derived state. Nonetheless, the ease of
removal of the tadpole has led to the suggestion that the
original tadpole evolved as an insertion in the lineage leading
to the anurans.
(6,8)
There have been many deviations from the usual anuran
life history, in which both eggs and tadpoles are aquatic.
(7,9)
Such deviations include gastric brooders, marsupial frogs,
arboreal frogs with foam nests, and multiple independent
occurrences of direct development.
(7,10,11)
Duellman and
Trueb
(7)
noted that the adoption of a terrestrial life style
correlates with the trend towards direct development. The term
``direct development'' has been employed in slightly different
contexts in the literature. Some authors include in this
definition nidicolous species; these hatch as non-feeding
tadpoles and undergo metamorphosis after utilising their
endogenous yolk supply. An anuran example of this type of
development is Nectophrynoides malcolmi.
(12)
Wake
(12)
con-
siders this species a direct developer since, although there is a
morphologically evident tadpole, it does not forage for food,
and is relatively non-motile. This classification is ecological
rather than morphological. We are concerned in this review
with the mechanistic basis of direct development, so we are
using a morphological definition. We consider only those
species that lack a morphologically apparent larval phase to be
BioEssays 23:233±241, ß 2001 John Wiley & Sons, Inc. BioEssays 23.3 233
1
Department of Biochemistry and Cell Biology, SUNY, Stony Brook,
NY 11794, USA.
2
Institute for Biological Sciences, National Research Council, Ottawa,
Canada.
3
Department of Biological Sciences, Duquesne University, Pittsburgh,
PA 15282, USA.
Funding agency: NSERC, Canada.
*Correspondence to: Richard Elinson, Department of Biological
Sciences, Duquesne University, Pittsburgh, PA 15282, USA. E-mail:
elinson@duq.edu
Review articles
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