摘要:

Summary(1) We have put forth the position that evolutionary sequences can be deduced by an analysis of fundamental developmental sequences. Such sequences are highly conserved within a group and ‘contain steps which are necessary to achieve a developmental fate’. The premise of our work then, is that such fundamental sequences do not arisede novotime and time again but can be traced back through their evolutionary history in organisms which contain portions of the sequence.(2) These highly conserved developmental sequences are in fact developmental constraints to evolution in as much as natural selection has not been able to discard them, but rather has utilized them in achieving evolutionary change.(3) We have demonstrated the ability to use developmental data by producing an evolutionary sequence for the origin of the vertebrates using the processes of neuralization and cephalization, the latter due primarily to the influences of the neural crest and epidermal placodes. The evolutionary sequence created, while not novel in structure, is distinct in that it was created solely by following a developmental sequence that is highly conserved among the vertebrates. The sequence is:(a) Chordamesoderm differentiates from the surrounding mesoderm and induces an overlying neural tube.(b) Through the influence of neuralizing morphogens, the neural tube differentiates into anterior (fore‐, mid‐ and hindbrain) and posterior (spinal cord) parts. Cephalization has begun.(c) Cephalization proceeds via the development of two new populations of embryonic cells, the neural crest, a derivative of the neural epithelium and the epidermal placodes, derivatives of the ectoderm immediately adjacent to the neural tube. These two populations contribute significantly to the subsequent development of the vertebrate head including the skeleton, connective tissues, cranial nerve and sensory organs.Sequence (a) occurs in the most primitive protochordates and is one of the differences between the chordates and deuterostome invertebrates. Sequence (b) occurred next leading to a protochordate with a differentiated central nervous system, but lacking most vertebrate head structures. Sequence (c) signalled the beginning of the true vertebrates or branchiates (after the branchial arches which all vertebrates' share) since the production of a neurocranium, viscerocranium, cephalic armour, teeth and cranial peripheral ganglia was only possible with the acquisition of this developmental step.(4) Current investigations into the cellular, molecular and biochemical basis of developmental sequences will allow biologists to test for the integrity of these developmental sequences and to test for their presence in the ontogenies of any species in question. These investigations will ascertain the validity of determining an evolutionary sequence based on a particular developmental process(es) as outlined in the presen

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1989.tb00672.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

Summary1. Polychaete sperm are divisible into ect‐aquasperm, ent‐aquasperm, and introsperm.2. Ect‐aquasperm are the commonest type of polychaete sperm and are considered plesiomorphic for the Polychaeta. Re‐evolution of ect‐aquasperm (as neo‐aquasperm) is, nevertheless, tentatively hypothesized for some Sabellida.3. In terms of ultrastructural studies of sperm in the investigated polychaete families, only ect‐aquasperm have been demonstrated for 16 families; only ent‐aquasperm for 3 families; ect‐ and ent‐aquasperm for 3; ect‐ and intro‐sperm for 2; ect‐, ent‐ and intro‐sperm for 1 family; and only introsperm for 11 families but investigations can only be regarded as preliminary. To date no family is known to have ent‐ and intro‐sperm only. Sperm ultrastructure has yet to be examined in the orders Magelonida, Psammodrilida, Cossurida, Spintherida, Sternapsida, Flabelligerida and Fauvelopsida.4. Much variation occurs in gross morphology, ultrastructure and configuration of the several components of ect‐aquasperm: acrosome, nucleus, mitochondria, and centrioles and associated anchoring apparatus. A 9 + 2 axoneme is constant.5. Group‐specific sperm structure has been demonstrated for the Nereidae (chiefly ect‐aquasperm), and for introsperm of the families Histriobdellidae, Questidae; Capitellidae, Spionidae and Protodrilidae. Species‐specificity of all classes of spermatozoa is well established.6. The very small size of ect‐aquasperm is correlated with production of large numbers of sperm as an adaptation to broadcast spawning. Simplicity of structure may relate more to conservation of materials than to hydrodynamics.7. Fertilization by ent‐aquasperm requires fewer eggs than in external fertilization and is accompanied by a tendency to lecithotrophy. Elongation of the nucleus and development of asymmetry are seen in several of the few known examples of ent‐aquasperm. Whether modifications are related to transfer or to other features, such as lecithotrophy, is uncertain.8. Evident multiple origins of polychaete introsperm contraindicate their value in establishing relationship between families, in contrast with their utility in groups such as decapod Crustacea.9. At the intrafamilial level polychaete introsperm have taxonomic and phylogenetic value, as seen in the Spionidae, Capitellidae, and Histriobdellidae, and are distinctive of each of these and other families.10. At higher taxonomic levels, the ultrastructure of the sperm of the oligochaetoid Questidae distinguishes this family from euclitellates, each class of which has its own distinctive subtype of the euclitellate introsperm.11. A number of categories of polychaete introsperm, with examples, are recognized: those which differ from ect‐aquasperm chiefly in elongation of the nucleus; approximately spheroidal, non‐motile sperm; sperm with superficially ‘primitive’ facies but in fact modified; and filiform flagellate or aflagellate sperm.12. Filiform development and extreme modification appear often to be related to transdermal insemination but may also be adaptive to packaging in spermatophores and seminal receptacles.13. Polychaetes exemplify multiple and pronounced parallel evolutionary modification of both reproductive biology and sperm structure which must have contributed subs

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1989.tb00673.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1989.tb00674.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

SummaryThe sheet‐runner continuum model of unilaminar encrusting colony growth is reassessed for cheilostome Bryozoa. It is concluded that the model does not adequately account for the existence of spatially predictable refuges from mortality, which can be selected by the larva at the time of settlement.A third end‐point category of colony form, named the spot colony, is recognized for species settling in small spatially predictable refuges and growing to small, early maturing colonies of determinate or semi‐determinate size. Similar colonies are reported from spatially restrictive substrates such as flexible algal fronds and single sediment grains on particulate seabeds.Runner growth is also reappraised. In some cases, uniserial growth may be regarded as a primary adaptation for growth in linear refuges, or on maze‐like or strongly three‐dimensional surfaces where multiserial growth is impossible, rather than as a general fugitive strategy adopted by competitively inferior forms.A revised classificatory model for encrusting growth is proposed. This consists of two continua, sheet‐ribbon‐runner and sheet‐patch‐spot. It is suggested that an improved ecological classification of encrusting growth might be framed as a series of coupled settlement/

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1989.tb00675.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1989.tb00676.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY