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ans. Analyses of such embryos suggest that some vegetal cells do not contribute to
any of the three germ layers and are instead nutritive yolk cells only. This fact,
combined with the evolutionary position of basal fish like bichirs and lampreys,
points to the conclusion that, while holoblastic cleavage is ancestral in vertebrates,
the particular form observed in amphibians, with all blastomeres contributing to
embryonic tissues, is derived. This conclusion is further bolstered by the observa-
tion that a maternally expressed homologue of vegT, which is crucial for early
amphibian endoderm development, appears to only be found in amphibians and not
other vertebrate species, such as mice, bichirs, lampreys, and teleosts (Takeuchi
et al. 2009 ).
The ancestral trait of having explicitly nutritive yolk cells in vertebrates may
have provided early embryos an evolutionarily advantageous ability to implement
cell division in the absence of yolk granules while nevertheless maintaining embry-
onic nutritive stores, an advantage that may have been maintained in meroblastic
cleaving embryos. Developing embryos are known to rely on exquisitely precise
cellular processes, such as cytoskeletal reorganization and the recycling of mem-
brane particles during cell division, and it is easy to imagine that the presence of
yolk particles may interfere with, or add variability to, this process. Selection
against such interference could be one cause of a transition to a meroblastic cleav-
age system in an animal’s lineage.
The inference of explicitly nutritive yolk cells in ancestral vertebrates may also
make it easier to understand precisely how meroblastic cleavage might evolve from
holoblastic cleavage. For example, fusion of yolky blastomeres into a single nondi-
viding mass is proposed to be the second of two changes that occurred leading to
evolution of the teleost embryo, the first being loss of bottle cells that are still pres-
ent near the beginning of gastrulation in more basal taxa (Collazo et al. 1994 ). Given
this, groups such as bowfins (Amia) and gars (Lepisosteus) (Ballard 1986a, b; Long
and Ballard 2001 ), which exhibit cleavage patterns that appear to be partially mero-
blastic (Fig. 4.14a), may be representative of ancestral transitional states along a
continuum from holoblastic to meroblastic cleavage.
An important correlation that has long been observed in the study of cleavage
pattern evolution is that meroblastic cleavage often correlates with large egg size
(Collazo 1996 ). Egg size correlation can be most clearly seen in amniotes where
large eggs and meroblastic cleavage are predominant, with the small embryos of
eutherian mammals and marsupials having returned to essentially holoblastic cleav-
age. Furthermore, as discussed above, the generally small eggs of amphibians
exhibit a likely derived form of holoblastic cleavage. In such cases, selected traits
such as differing degrees of reliance on egg nutritive stores may underlie the corre-
lation between cleavage type and egg size, although other explanations have been
proposed (Collazo 1996 ).
Teleosts constitute a major exception to this correlation, as there appears to be a
sharp decrease in egg size in their stem phylogenetic lineage (Collazo 1996 ).
Conversely, the Puerto Rican tree frog, Eleutherodactylus coqui, has an egg approx-
imately 20 times the size of Xenopus laevis but in spite of this enormous size main-
tains a holoblastic cleavage pattern. How these particular species may escape the
A. Hasley et al.