The 26 recognized minor phyla comprise aberrant clades, as most of
them terminate as blind offshoots. Untied from the discussion on
their phylogenesis of minor phyla, this book is largely devoted,
for the first time, to aspects of reproduction and development in
minor phyletics. The minor phyla are not as speciose (1,795
species/phylum) as the major phyla (157,066 species/phylum) are.
The accumulation of deleterious genes causes inbreeding depression
among progenies arising from parthenogenesis, clonal multiplication
and selfing hermaphrodites. The reason for the limited species
diversity in minor phyla is traced to (i) eutelism in 65.7% of
minor phyletics and (ii) existence of 21.6% clonals, (iii) 6.4%
parthenogens and (iv) 1.2% selfing hermaphroditism. Gonochorism
obligately requires motility to search for a mate. The combination
of low motility and gonochorism from Placozoa to hemocoelomatic
minor phyla has limited diversity to < 1,000 species. Over 19%
of minor phyletics are hermaphrodites. With the need to manifest
and maintain dual sexuality, fecundity of hermaphrodites may be
reduced to 50% of that in gonochores. Adopting an array of
strategies, < 100 hermaphrodites are selfers. In eutelics,
mitotic division is ceased in somatic cells after hatching. For the
first time, the prevalence of eutelism has been brought to light in
numbers of all the six pseudocoelomate phyla and priapulids.
Eutelism limits fecundity to 30-300 eggs in free-living
pseudocoelomates, priapulids and possibly other hemocoelomates. In
them, sperm production is less than that of egg production; as a
result, a large fraction of their eggs is sterile. With a high
proportion of non-eutelic gametic cells (35%), Nematoda and
possibly Nematomorpha and Acanthocephala are more fecund than
rotifers, in which the proportion is 15%. Briefly, the reasons for
the limited species diversity in minor phyletics are traced to
eutelism, parthenogenesis and clonal multiplication. Surprisingly,
parthenogenesis and clonal multiplication mutually eliminate each
other. This is also true of hermaphroditism and parthenogenesis.
However, clonal multiplication is prevalent from structurally
simplest Placozoa to the most complex Ascidiacea, except in
pseudocoelomates and hemocoelomates. A limited number of cells and
cell types, and the consequent structural simplicity facilitate
manifestation of parthenogenesis in pseudocoelomates and parasitism
in Mesozoa, Myxozoa, 59% of Nematoda, Nematomorpha, Acanthocephala
and Pentastomida. Despite hermaphroditism, Bryozoa (5,700 species)
and Ascidiacea (3,000 species) are speciose among minor phyla. For
the first time, the importance of fusion of fragments or colonies -
an event equivalent to gamete fusion at fertilization - is
recognized as a source of new gene combination. Besides, the
colonies in these minor phyla degenerate and regenerate more or
less regularly. Only the fittest degenerated colonies may be
rejuvenated and regenerated.
General
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