Affordable Access

deepdyve-link
Publisher Website

Phylogeny and evolutionary history of the amniote egg.

Authors
  • Starck, J Matthias1
  • Stewart, James R1, 2
  • Blackburn, Daniel G3
  • 1 Department of Biology, Ludwig-Maximilians-University Munich, Planegg-Martinsried, Germany. , (Germany)
  • 2 Department of Biological Sciences, East Tennessee State University, Johnson City, Tennessee, USA.
  • 3 Department of Biology, Trinity College, Hartford, Connecticut, USA.
Type
Published Article
Journal
Journal of morphology
Publication Date
May 15, 2021
Identifiers
DOI: 10.1002/jmor.21380
PMID: 33991358
Source
Medline
Keywords
Language
English
License
Unknown

Abstract

We review morphological features of the amniote egg and embryos in a comparative phylogenetic framework, including all major clades of extant vertebrates. We discuss 40 characters that are relevant for an analysis of the evolutionary history of the vertebrate egg. Special attention is given to the morphology of the cellular yolk sac, the eggshell, and extraembryonic membranes. Many features that are typically assigned to amniotes, such as a large yolk sac, delayed egg deposition, and terrestrial reproduction have evolved independently and convergently in numerous clades of vertebrates. We use phylogenetic character mapping and ancestral character state reconstruction as tools to recognize sequence, order, and patterns of morphological evolution and deduce a hypothesis of the evolutionary history of the amniote egg. Besides amnion and chorioallantois, amniotes ancestrally possess copulatory organs (secondarily reduced in most birds), internal fertilization, and delayed deposition of eggs that contain an embryo in the primitive streak or early somite stage. Except for the amnion, chorioallantois, and amniote type of eggshell, these features evolved convergently in almost all major clades of aquatic vertebrates possibly in response to selective factors such as egg predation, hostile environmental conditions for egg development, or to adjust hatching of young to favorable season. A functionally important feature of the amnion membrane is its myogenic contractility that moves the (early) embryo and prevents adhering of the growing embryo to extraembryonic materials. This function of the amnion membrane and the liquid-filled amnion cavity may have evolved under the requirements of delayed deposition of eggs that contain developing embryos. The chorioallantois is a temporary embryonic exchange organ that supports embryonic development. A possible evolutionary scenario is that the amniote egg presents an exaptation that paved the evolutionary pathway for reproduction on land. As shown by numerous examples from anamniotes, reproduction on land has occurred multiple times among vertebrates-the amniote egg presenting one "solution" that enabled the conquest of land for reproduction. © 2021 The Authors. Journal of Morphology published by Wiley Periodicals LLC.

Report this publication

Statistics

Seen <100 times