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VERTEBRATE REPRODUCTION.

Sexual & seasonal factors, types, care of young, fertilization in fish, reptiles, birds, mammals. Outline.

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Vertebrate Reproduction: An Outline
I. Introduction.
II. General features of vertebrate reproduction.
A. Sexual reproduction.
B. Seasonal reproduction.
C. Polygamy versus monogamy.
D. Oviparity, ovoviviparity, and viviparity.
E. Caring for the young.
III. Phylum Chordata.
A. Amphioxus.
IV. Superclass Agnatha.
A. Lamprey.
V. Superclass Gnathostomata.
A. Elasmobranchii.

The evolution of viviparity in lizards. Different vertebrate species may also be classified as beingoviparous, ovoviviparous, and viviparous. A textbook. With polygamy, one individual mateswith more than one partner. Proceedings of the National Academy of Sciences of the United States of America 92:8 92-8 99; 1995, August 29. The crocodiles (Crocodilia) are perhaps the most advanced;crocodiles represent an older offshoot of the same "stock from which modernbirds were derived (12:276)." The reptiles were probably the first organisms to fully address thedifficulties associated with terrestrial reproduction. Third edition. Amphibia.VII. While fertilization among the teleosts is usually external, thefishes can also be viviparous. BioScience 43:742-751; 1993, December. The anatomical organization of a typical vertebrate bears certainsimilarities to amphioxus. The life of the vertebrates. J., Jr.; Dubois, D. 7. Male amphibians oftenmigrate to their breeding ponds first. D. Guillette, L. Most vertebrate species have separate sexes. T. For one, the front end of the nervous system isdifferentiated into an elaborate brain. According to hormonal cues, primary follicles develop intomature follicles. Among the Metatheria, the order Marsupialia issometimes considered less advanced than other mammals. Theactinopterygian fishes eventually gave rise to modern bony fishes; someforms of the crossopterygians led to the vertebrate class Amphibia(12:161). Most male reptiles have developed specialorgans of copulation. Saunders Company; 1968.1 . Theoffspring of Zoarces, Gambusia, and Poecilia species develop within theovary (12:215). Fertilization in mammals. A. In fact, the evolution of the mammary glandrepresented a "fundamental advance over the reptilian condition (12:567)."This extended period of care has ultimately facilitated the development ofcomplex behaviors and social structures (2:483). In most vertebrates, this enlarged brain is encased by aprotective skeletal thickening. During embryonic life, the testes of all mammalian species areinitially located in the abdomen. The behaviors may then keep the couple together during theincubation of their eggs or feeding of their offspring. P.; Lombardi, J. Eggs may be left to develop on theirown, or they may be cared for by one or both parents. Courtship behaviors--involving, forinstance, song or plumage--help bring individual birds of opposite sextogether. Reproductive physiology of vertebrates. Teleosts.VI. Development ofthe embryos occurs free in the water (12:46). This coiled, thick-walled vas deferens secretes amaterial that aggregates the sperms into spermatophores. In ovoviviparous organisms,the embryos develop inside their egg membranes within some part of thefemale urogenital system. Theseopen into a urodaeum by an erectile papilla. Thesespermatophores are then picked up by the female's cloaca, and fertilizationtakes place in the oviduct (12:252-254). In some catfish, the young develop within a parent's mouth.Moreover, male sea-horses possess special pouches for their offspring. Two important Gnathostomata groups which arose in thelate Devonian include the cartilaginous fishes (Chondrichthyes) and thebony fishes (Osteichthyes) (12:114). In thesespecies, the eggshell may be reduced to a thin membrane. B. These organismspossess both testes and ovaries. In most species, however, the testesdescend into the scrotum and are maintained below body temperature. Perhaps the most distinguishing feature of the chordatesis their notochord. Superclass Agnatha. These structures facilitate fertilization. During ovulation, the follicle ruptures and the egg istransported through the oviducts. An explanation of the high strain diversity of a self-fertilizing hermaphroditic fish. Groups of coresidentadults, or families, provide an environment whereby offspring can be bothsocialized and educated (3:8 92-8 93). Inthe classes, Aves and Mammalia, offspring may continue to interactregularly with their parents on into adulthood. In contrast to Agnatha, the superclass, Gnathostomata, does have ajaw mouth. Z. Phylum Chordata. Superclass Gnathostomata. Althoughmost reptiles are oviparous, ovoviviparous lizards and snakes retain theireggs within their oviducts until the young are ready to hatch. The upper part of the oviduct secretes mainly albumen; whereas,the lower part secretes mucus. In addition, theyalso ensure that the eggs are deposited into the lampreys' "nest (12:85)." After about 3 weeks, the ammocoete larva hatch. For example, a variety of fossilized dinosaur nests have beenfound worldwide (7:59). Young lizards can be bearable. This organism features separate sexes andexternal fertilization. The most successful modern forms, however, are the lizards and snakes(Squamata). A shell provides theseeggs with both physical support and protection against desication. The spermare flagellated and motile. Inaddition, reptile embryos are also provided with special embryonicmembranes, the amnion and allantois, and a large quantity of yolk. Seasonal reproduction. Among thevertebrates, a diverse array of reproductive mechanisms have evolved.These include both sexual and asexual fertilization events. During copulation, theproctodaea of the male and female are everted and pressed together. S. Marsupial young arenot nourished by a complex placenta. Spermatophores are transmitted to the female by a large pair ofclaspers. The descendants of the acanthodianscan be broadly divided into two main groups: The actinopterygian, or ray-fin fishes, and the crossopterygians, or lobed-fin or lung fishes. Among the different eutherians,there are large variations in the extent to which fetal and maternal bloodare separated (12:431). At present though, it is exhibited bysome species of all vertebrate classes, with the exception of Aves and thejawless fishes (4:742). In male elasmobranchs, the sperm iscollected by vasa efferentia which lead into the reproductive portion ofthe mesonephros. Simultaneous hermaphroditism occurs whenan organism's testes and ovaries mature and produce gametes at the sametime. In these mammals, the fertilizedegg develops multiple cell layers. For the mostpart, both male and female vertebrate gonads are paired organs. In addition, just like birds, crocodiles may alsobuild nests. Yolk lipids andproteins are synthesized in the liver and transferred to the egg throughthe follicle cells. Perhaps the best-known example ofconsecutive hermaphroditism occurs in the wrasse, Labroides dimidiatus.The males of this species of marine cleaner fish typically possess a haremof several females. They then use their vocal apparatusto attract mates. Elasmobranchii. Vertebrate species may also have multiple modes of reproduction. The mode of reproduction firstevolved among the fishes (11:276). They are nearly all carnivores or scavengers, and include thesharks, skates, and rays. 8. Blüm, V. Most Amphibiaspecies, however, are not completely terrestrial. The ammocoete larva lives in fresh water buried in mud. Ecology 76:593- 595; 1995, March. The femalewrasse can change herself into a "new" male in a matter of days (1:31-32). Hence, in young ammocoetes the organcontains both oocytes and spermatocytes, and is therefore hermaphroditic(12:85). Mechanisms have evolved which not only furnishembryos with nutrients, but facilitate gas exchange and remove wastes aswell. During the late Devonian, a group of freshwater fish eventually gaverise to the vertebrate class, Amphibia. B. Metatheria. Both male andfemale birds may also collaborate on nest building (12:379-385). Reptilia.VIII. The lips of the male cloaca areunited to form a narrow-penis-like tube, and the female's cloacal lips areenlarged. Hui-Chen, L.; Dunson, W. From there, it passes upinto the oviduct (12:357-358). In addition, most species have varying degrees ofplacentation. Aves.IX. New York, NY: Springer-Verlag; 1986. Moreover,offspring may be produced by oviparous, ovoviviparous, and viviparousprocesses. 4. Theamnion and allantois permit gas exchange and the storage of waste products. Although some bird species are polygamous, the great majorityform pairs which last throughout the breeding season. Vertebrate species mate in different ways. Female crocodiles not only guard their nests, they may alsoprotect their offspring both during and after hatching (12:285-286). The "great" subphylum, Vertebrata, actually belongs within the phylumChordata (12:75). Some of these cells develop into atrophoblast which protects the embryo from immunological attack. Caring for the young.III. General features of vertebrate reproduction. Second, the vertebrate notochord consistsof bone and cartilage: Groups of large cells secrete and are separated bya matrix of protein chondroitin sulphate. These areoften produced in large numbers. Perhaps the most important advantage of viviparity is the head startwhich it provides young organisms (8:38). Ovoviviparity isknown to occur in the following vertebrates: Elasmobranchii, Teleostei,Amphibia, and Reptilia (1:237). A. H.; Cree, A. Numerous small yolky eggs are produced. The most primitive reptiles are the tortoises and turtles (Chelonia). Amphioxus.IV. The yolk provides the reptile embryo with adequate nutrition. Reptiles can be both oviparous, ovoviviparous, and viviparous. Moreover, this brain containsspecial receptors (e.g., the nose, the eye, and the ear). This bony matrix both supportsand protects the spinal cord (12:78). Perhaps the earliest life form to develop were thereptiles. A. This consists of several membrane layers and a large allantois.In contrast, the endotheliochorial placenta of carnivores and thehemochorial placenta of the higher primates consists of fewer membranouslayers, and provides for more efficient exchange (12:567). It enabled vertebrates to become moreactive carnivores. Among the mammals, the processes also include an advancedmechanism for nurturing the young. In these species, the young are either nourished byyolk or by maternally secreted embryotrophe. Male frogs cling to the back of female frogs by means ofan extra digit on their front appendages. The fertilized ovum may eventuallyimplant in the uterus (9:46-53). Emlen, S. Reflections on the evolution of piscine viviparity. In order to fly, these organisms must have minimal weight. In many species, larval forms occur. Over time, theseorganisms grow from about 7 millimeters in length to over 17 millimeters.Eventually, the ammocoetes undergo a complete metamorphosis into adultlampreys and migrate to the sea (12:1 ). The teleost reproductive systemmay be characterized as being completely separate from the excretoryapparatus (see figure 7.1 ). Instead, they have a relatively shortgestation and give birth to very immature young (1:273). The teleosts have a diverse range of breeding habits. Thespermatophores are then stored in a broader ampulla which leads to a"forwardly directed blind diverticulum," or sperm sac (12:137). Fertilization inall modern reptiles is internal. Marsupials. A. Although such embryos are not supplied withmaternal nutrients, gases and water may diffuse through the egg membranes.The mother's body also gives the embryos protection. The testes consists of a pair of large sacswhich open into the base of the urinary ducts. Only the chimaeras (i.e., Raja) and a few families of sharks(e.g., Scyliorhinus) lay eggs. Sexual reproduction. Guillete, L. This bundle of nerve fibers is surrounded by a layerof gelatinous material, and wrapped in a notochordal sheath (12:35-36).The lancelet, Amphioxus lanceolatus, represents a living example of achordate (see figure 2.1). This bonding may endure throughout the species'lifespan (1:29). The biting jaw developed about 395 million years ago, towardsthe end of the Silurian period. These offspringcrawl to a pouch, become attached to the mother's teats, and are nourishedprimarily by milk (12:424-426). However, both male and female lamprey possesscertain modifications of the cloaca. Sperm pass from the vasa deferentia into theurodaeum, and eventually passed along to the female (12:285-286). Although female birds have two ovaries, their eggs are so large thatonly one ovary actually develops (see figure 15.44). Polygyny occurs when the male mates withseveral females; polyandry involves a female mating with several males. Theactual binding of a sperm cell to an egg is species-specific: Inpractically all organisms, the egg's sperm receptors only recognizespecific sperm cells (1 :1988). Inmonogamous species, individuals mate with only one sexual partner.Monogamous relationships may be characterized by strong pair bondingbetween individuals. Fertilization occurs when the nuclei of anegg and a sperm cell fuse to form a zygote. These immatureamphibia usually occupy different habitats than the corresponding adults.Tadpoles, for example, may live in shallow waters and be herbivorous, ormay be nektonic and live on plankton (12:252-254). Mammalia. In some cases, such as the reptiles and birds,specialized reproductive processes represent an adaptation to a newenvironment. 1. The gametes areshed into an atrium, and then released through an aperture. Conclusion. However, the subphylum also has certainadditional features. Inaddition, the sand goby (Pomatoschistos minutus) builds a structure whichresembles a nest. A. Sexual reproduction involves the combination of sperm and eggs. In viviparouselasmobranchs, the oviduct serves as the "uterus (12:138)." Some speciesare also aplacental. Oxford, United Kingdom: Clarendon Press; 1981.----------------------- 8 Perhaps the most striking characteristic of amphibians though istheir life cycle. Othercells develop into the embryo itself. Scientific American 259:78-84; 1988, December.11. Obviously, different vertebrate species employ a wide range ofreproductive strategies. Most species areviviparous. Sexual reproduction ensures thecombination of different genotypes. Elasmobranchii, or "plate-gilled" fishes, are a widespread group(12:137). J., Jr. Prostaglandins, oviductal function, and parturient behavior in nonmammalian vertebrates. Taken together, these highly variablestrategies provide diverse vertebrate species with an optimum chance ofproducing the next generation. Carter, G. These receptorsgive vertebrates the ability to respond to more varied aspects of theirenvironment. 5. B. Van Tienhoven, A. E. Natural History 1 3:34-39; 1994, January. Relatively complex mechanisms for nurturing offspring occur in theAves and Mammalia classes. A. For such organisms, the period of gestationmust have been very short. During late Ordovician, Silurian, and Devonian times, a group of bonyfishes known as acanthodians evolved. Vertebrate Reproduction: An OutlineI. For example, prosimians have an epitheliochorialplacenta. Shine, R. B. M. 2. The two primary types ofmating include polygamy and monogamy. In contrast, viviparous species provide for their embryos' needs upuntil the time of birth. Vertebrates also provide variable levels of care for their young. Wassarman, P. Structure and habit in vertebrate evolution. These eggs are usually provided with a sticky coveringwhich keeps them attached to each other, and also to stones and weeds. Introduction.II. Through divergent evolution, a variety of different creatures evolvedfrom the amphibians. 6. Gametes accumulate in the lamprey's coelum, and escape through shortchannels leading to the lower end of the kidney duct. Finally, inviviparous species, the allantois, the yolk sac, or some combination ofthese membranes may develop into a placenta (12:285-286). Seattle, WA: University of Washington Press; 1967. Therefore, individualorganisms typically alternate between periods of reproductive activity andinactivity. The lamprey, for instance, has two life stages. Polygamy versus monogamy. For the most part, oviparouselasmobranchs generally consist of small animals which live on the bottomnear shore; whereas the larger, pelagic sharks are more likely to beviviparous (12:138). Incontrast, vertebrate eggs are generally surrounded by follicle-formingnurse cells. Its gonads consist of hollow segmental sacs withno common duct. When the male is removed from the harem, however, itcan be replaced by either another male or the dominant female. Withregard to reproduction, the class Mammalia, is perhaps most distinguishedby its care of the young. In contrast, theadult lamprey has a sucking mouth and parasitizes other fish. A pair ofsiphon-like sacs then wash extruded sperm down the groove. Osteichthyes. In contrast, consecutivehermaphroditism occurs when the different sets of gonads mature insequence, one before the other. An evolutionary theory of the family. Philadelphia, PA: W. Of all the vertebrate species, those belonging to the superclass,Agnatha, do not have jaws. Modern fish generally belong to the class Osteichthyes and thesubclass Actinopterygii (ray fins). Although reptiles are not known for parental care, some forms dobuild nests. American Zoologist 32:276-293; 1992, April.12. The processes involved in amphibian metamorphosis are controlled byhormones. These changes may involve practically every part of the body.The resorption of such larval structures as gills, tail, and beak, makesway for the development of limbs, eyelids, and lungs (12:252-254). Spermis ejaculated directly into the female urodaeum. 3. In addition, most early fishes havebeen nearly completely replaced by members of the infraclass, Teleostei(bony fishes) (see figure 8.3) (12:194). The eggs are then depositedeither as compact masses or in strings. In contrast, the ovariesare elongate, closed muscular sacs which are continuous with the oviducts. These modified pelvic fins form grooved, scroll-like organswhich can be inserted and anchored within the female oviduct. In contrast, the infraclass Eutheria can be characterized by theplacental development of its offspring. The vertebrate class, Aves, is characteristically associated with theability to fly. These phases produce regular changes in organisms' gonads, andare often attuned to certain environmental changes. As in amphioxus,fertilization is external. With the evolutionary development of internal fertilization, however,lengths of gestation could increase (5:R854). Additionally, in the male newt,special pelvic and cloacal glands form spermatophores. For example, in thehigher latitudes, vertebrate reproductive phases are generally coupled withthe changing of the seasons (1:23-24). These organisms stillhave to return to the water to breed; different amphibian species arecapable of both external and internal fertilization. American Journal of Physiology 26 (5 Part 2):R854-R861; 1991, May. The eggs may have only remained in the oviductsfor only a few hours or days (5:R854). Literature Cited 1. Vertebrate Reproduction The preservation of species requires reproduction. Incontrast, the ovaries of mammalian species generally remain suspended fromthe body wall. Therefore, bird offspring develop outside the body (12:323). Some teleost species are also hermaphroditic. 9. It has been hypothesized thatancestral vertebrate reproduction consisted of oviparity, or egg-laying,with external fertilization. Thelamprey's gonads occur as unpaired ridges medial to the mesonephros.Differentiation of this gonad into either eggs or sperm occurs relativelylate in the lamprey life cycle. Lamprey.V. Pelagic eggs areusually provided with an oil globule which keeps them afloat. The class Mammalia can be further divided into the infraclasses,Metatheria and Eutheria. 1. Norell, M. C. In male vertebrates, the testesproduce spermatozoa; in females, the ovaries produce eggs. Genetic variation isthe "raw material" of evolution (6:593). Eutheria.X. Oviparity, ovoviviparity, and viviparity. The egg then passes down the ciliated, muscularoviduct. In some cases, theserelationships may last even longer. Forexample, southeastern Australia's nearly limbless burrowing lizard,Bougaineville's skink, is both "egg laying and live bearing (8:37)." Theorganism generally exhibits viviparity in the colder parts of its range. In somespecies, fertilization is external. Desmersal eggs, on the other hand, sinkto the bottom. One such teleost, the tropical and subtropical estuarine fish,Rivulus marmoratus, is self-fertilizing (6:593). The zygote then undergoes aseries of cell divisions and differentiations, and ultimately forms a newindividual (1:19). Subsequent divergent evolutionary lines would eventually includethe birds and mammals (12:276). Then, following copulation and ovulation, fertilizationcan take place in the uterine tubes. Origins of the feathered nest. Bird testes connect to a long epididymis and vas deferens. The breeding habits of birds involve elaborate systems of mutualrelations. The genital systems of these organisms is highlyspecialized for internal fertilization (see figure 5.24). They "swim" in fluids produced by specializedglands, and eventually leave the body via the spermiducts (1:19). Young, J. Theeggs of oviparous species are always laid on land. In two orders of teleosts, cyprinodonts andperciformes, there are species which engage in internal fertilization. Most vertebrates reproduce periodically. The different vertebrate classes additionally exhibitconsiderable variation in the amount of care which is provided to theyoung. Hormonal mechanisms ensure that sufficientcalcium is mobilized from the bones to the blood, and made available foregg-laying (12:357-358). Moving from pool to pool, the fishgradually became more adapted to a terrestrial environment. Wourms, J. This protected area is guarded and aerated by the male.Other variations on protection of the young are found in catfish and sea-horses. Vertebrate reproduction. Natural History 1 4:58- 61; 1995, June.

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