Reproduction and Development in Minor Phyla

T. J. Pandian

• Publisher: CRC Press
• Year: 2021
• Topic: Biological Sciences
• eBook ISBN:
  9781000284584
• Subtopic: Biology
• Index: Biological Sciences

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.

1

General Introduction

Introduction

The minor phyla comprise aberrant clades that are not usually considered in the mainstream of evolution. In the phylogenetic tree, most of them terminate as blind offshoots, albeit a few are regarded as a link between two or more major phyla; for example, the velvet worm Onychophora provides a vital link for arthropod evolution. Hyman (1951a, b, 1959) has brought out excellent accounts for 16 minor phyla among the then known 22 aberrant clades. As new taxa are being continuously erected, four minor phyla (Placozoa, Myxozoa, Loricifera, Cycliophora) are now added. Pogonophora is excluded and is now included in polychaetes (see Pandian, 2019). Presently, the aberrant clades are classified into 26 minor phyla. Figure 1.1 shows perhaps for the first time the pictures assembled for representative species from 26 minor phyla. There are a large number of hypotheses and theories describing the phylogenetic relations and affinity between two or more phyla. Discussions, and debates are ongoing on them. A vast majority of publications, reviews, chapters, books and book series elaborate the relationships and affinity between these phyla. As there is more than adequate information on this theme, this account shall not describe or discuss the phylogenetic relations. But it is rather devoted to focus on other aspects, which are not thus far adequately recognized.

Taxonomic Distribution

The minor phyla are not as speciose (1,795 species/phylum, Table 1.1), as the major phyla are (157,066 species/phylum). In six of them, the species number is < 100 (Placozoa, Loricifera, Cycliophora, Gnathostomulida, Priapulida, Phoronida), a few hundred in a dozen of them (Mesozoa, Gastrotricha, Kinorhyncha, Nematomorpha, Sipuncula, Echiura, Onychophora, Pentostomida, Entoprocta, Brachiopoda, Chaetognatha, Hemichordata) and ranges from 1,000 to 3,000 in another six of them (Myxozoa, Nemertea, Rotifera, Acanthocephala, Tardigrada, Urochordata). Only Nematoda (27,000 species) and Bryozoa (5,700 species) are speciose. Further, a few phyla are represented by two-three species (e.g. Placozoa, Cycliophora) and two families (e.g. Priapulida, Onychophora). Some of these values may be compared with the least speciose Echinodermata (7,000 species, Pandian, 2018) and > million speciose Arthropoda (Chapman, 2009). Presently, the minor phyla comprise 46,687 species. They are grouped into (i) Aorganomorpha (a new term coined to indicate the lack of organ, in line with acoelomorpha with no coelom), (ii) Acoelomorpha, (iii) Pseudocolomata, (iv) Hemocoelomata, (v) Lophophorate schizocoelomata and (vi) Eucoelomata. The number of phyla within each of these groups and the number of species in each phylum are shown in Fig. 1.2. Among them, pseudocoelomates with six phyla, each with the highest mean species number of 5,251 per minor phylum, have generated more often aberrant clades. Surprisingly, they are all eutelic, i.e. after hatching, mitosis is ceased in their somatic cells, a feature which has not been adequately recognized. Notably, major phyla have no representative from Pseudocoelomata.

FIGURE 1.1
A. Placozoa: Trichoplax adhaerens (Eitel et al., 2011), B. Mesozoa: Dicyema japonicum. C. Actinospore of Myxozoa. D. Loricifera: Rugiloricus, E. Cycliophora: Symbion pandora, P. Nemertea: Ototyphonemertes sp (after Botosaneanu, 1986), G. Gnathostomulida: Gnathostomula jenneri (drawn from Biocyclopedia), H. a bdelloid rotifer, I. Gastrotricha: Chaetonotus sp (from Biology Discussion), J. Kinorhyncha: Echinoderes sp (free hand drawing from Bumblebee.org), K. Nematoda: Ascaris lumbricoides, L. Nematomorpha: Gordius (from Biology Discussion), M. an acanthocephalan (from NC State University), N. a priapulid (after Kingsley, 1884), O. Sipuncula: Sipunculus nudus (drawn from Maiorova and Adrianov, 2017), P. Echiura: Echiurus (drawn from Biocyclopedia), Q. a tardigrad (from New York Times), R. Onychophora: Peripatus (drawn from BioScience Media), S. Pentastomida: Linguatula serrata (drawn from Saari et al., 2019), T. a phoronidan (drawn from Spektrum.de), U. Entoprocta: Urnatella (from Biology Discussion), V. a bryozoan (drawn from Academic Web Site), W. Brachiopoda: Lingua (drawn from Udo Savalli, Arizona University), X. Chaetognatha: Sagitta elegans (from New World Encyclopedia), Y. Balanoglossus Z. solitary ascidian (all others are free hand drawing from Hyman, 1959).

TABLE 1.1
Number of phyla and approximate species number in major (from different sources) and minor phyla (from this book)

Major Phylum	Species (no.)	Minor Phylum	Species (no.)
Aorganomorpha
Porifera	8553	Placozoa	3
Cnidaria	10856	Mesozoa	150
Acnidaria	166	Myxozoa	2200
Acoelomorpha
Platyhelminthes	21810	Loricifera	34
Turbellaria	5500	Cycliophora	2
Non-Turbellaria	16310	Nemertea	1300
		Gnathostomulida	100
Psuedocoelomata
Annelida	16911	Rotifera	2031
Echinodermata	7000	Gastrotricha	813
		Kinorhyncha	200
		Nematoda	27000
		Nematomorpha	360
		Acanthocephala	1100
Hemocoelomata
Arthropoda	1166984	Priapulida	19
Crustacea	54384	Sipuncula	160
Non-crustacea	1112600	Echiura	230
Mollusca	118451	Tardigrada	1047
		Onychophora	200
		Pentastomida	144
Lophophorata
		Entoprocta	200
		Phoronida	23
		Brachiopoda	391
		Bryozoa	5700
Eucoelomata
Vertebrata	62869	Chaetognatha	150
		Hemichordata	130
		Urochordata	3000
Major phyla total	1413600	Minor phyla total	46687
Grand total	1460287
157,066 species/major phylum; 1,795 species/minor phylum

FIGURE 1.2
Number of phyla and species within the major dads of minor phyla. Data are drawn from Table 1.1.

Phylogenetic Cladograms

Enormous efforts have been made to construct phylogenetic cladograms to track the history of animal evolution. Based on structural and embryological features, zoologists have traced the animal evolution from simple to complex animals through gradual steps and bridging links, although they are very much aware that parasitism, for example, reverses the course from a complex to simple form, as in parasitic rhizocephalic (e.g. Sacculina carcini) crustaceans (Pandian, 2016). For classification, the conventional cladogram considers the absen...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Preface to the Series
• Preface
• Acknowledgements
• Table of Contents
• 1. General Introduction
• Part A: Aorganomorpha
• Part B: Acoelomorpha
• Part C: Pseudocoelomata
• Part D: Schizocoelomata: Hemocoelomata
• Part E: Schizocoelomata: Lophophorata
• Part F: Eucoelomates
• 27. References
• Author Index
• Species Index
• Subject Index
• Author’s Biography