The origin of Metazoa: a transition from temporal to spatial cell differentiation.
For over a century, Haeckel's Gastraea theory remained a dominant theory to explain the origin of multicellular animals. According to this theory, the animal ancestor was a blastula-like colony of uniform cells that gradually evolved cell differentiation. Today, however, genes that typically control metazoan development, cell differentiation, cell-to-cell adhesion, and cell-to-matrix adhesion are found in various unicellular relatives of the Metazoa, which suggests the origin of the genetic programs of cell differentiation and adhesion in the root of the Opisthokonta. Multicellular stages occurring in the complex life cycles of opisthokont protists (mesomycetozoeans and choanoflagellates) never resemble a blastula. Here, we discuss a more realistic scenario of transition to multicellularity through integration of pre-existing transient cell types into the body of an early metazoon, which possessed a complex life cycle with a differentiated sedentary filter-feeding trophic stage and a non-feeding blastula-like larva, the synzoospore. Choanoflagellates are considered as forms with secondarily simplified life cycles.
Mikhailov
Kirill V
KV
Belozersky Institute for Physicochemical Biology, Lomonosov Moscow State University, Moscow, Russian Federation.
Konstantinova
Anastasiya V
AV
Nikitin
Mikhail A
MA
Troshin
Peter V
PV
Rusin
Leonid Yu
LY
Lyubetsky
Vassily A
VA
Panchin
Yuri V
YV
Mylnikov
Alexander P
AP
Moroz
Leonid L
LL
Kumar
Sudhir
S
Aleoshin
Vladimir V
VV
D000818:Animals; D005075:Biological Evolution; D002454:Cell Differentiation; D005796:Genes; D008018:Life Cycle Stages; D008954:Models, Biological; D013997:Time Factors