157and reptiles). Convergent evolution of meroblastic cleavage is further supported by
differences in early development between the various lineages that undergo mero-
blastic cleavage (Collazo 1996 ). Independent evolution of meroblastic cleavage
appears to reflect a selective advantage. Once arisen within a lineage, meroblastic
cleavage is typically not lost, again consistent with an evolutionary advantage. A
notable exception to this pattern is the inferred reversion of cleavage pattern within
Amniota, from meroblastic to holoblastic as found in eutherian mammals and
marsupials.
In amphibian holoblastic cleavage, like that observed in the model vertebrate
Xenopus, all blastomeres eventually contribute to one of the three germ layers. The
ancestral nature of holoblastic cleavage is largely responsible for the widely held
assumption that amphibian-like cleavage represents the ancestral form of holoblas-
tic cleavage in vertebrates. However, evidence from bichir (Polypterus), a basal acti-
nopterygian (ray-finned fish), and lamprey (Lampetra japonica), a basal vertebrate,
challenges this view (Takeuchi et al. 2009 ). Vegetal cells in embryos of these organ-
isms do not express mesodermal or endodermal markers as in the case in amphibi-
Fig. 4.13 Independent appearance of meroblastic cleavage in various vertebrate phylogenetic lin-
eages. Phylogenetic tree of lineages from sea urchin to vertebrates, showing that meroblastic cleav-
age (black rectangles) arose multiple times and independently within these lineages. Diagram
reproduced from Collazo et al. ( 1994 ), with permission
4 Vertebrate Embryonic Cleavage Pattern Determination