(a) Explain asexual and sexual modes of reproduction in Paramecium. (20 marks) (b) Write general features of egg laying mammals…

(a) Asexual and sexual reproduction in Paramecium

Paramecium shows nuclear dimorphism: a large macronucleus that controls metabolism and one or more small micronuclei used in sexual processes.

Asexual: binary fission. The micronucleus divides by mitosis and the macronucleus elongates and divides by amitosis. A constriction forms across the middle, the cytoplasm divides transversely and the two daughters separate, each with a macronucleus and a micronucleus. The anterior daughter keeps the old oral groove, and the posterior one forms a new one. Repeated fission gives a clone.

Sexual: conjugation (P. caudatum), between two individuals of compatible mating types.

1. The conjugants unite by their oral surfaces and a cytoplasmic bridge forms; the macronucleus of each breaks down.
2. Each micronucleus divides meiotically into four haploid nuclei, of which three degenerate.
3. The survivor divides mitotically into a stationary and a migratory pronucleus.
4. The migratory pronuclei are exchanged across the bridge, and each fuses with the stationary pronucleus of the partner to form a diploid synkaryon.
5. The exconjugants separate. Each synkaryon divides three times into eight nuclei; four become macronuclei and the rest are micronuclei, of which only one persists. Two fissions then give four small paramecia from each exconjugant.

Autogamy (P. aurelia) is self-fertilisation in one cell: the micronuclei undergo meiosis, one haploid nucleus survives and divides into two that fuse, and a new macronucleus forms from the synkaryon while the old one disintegrates.

Conjugation is not multiplication, since two individuals give two exconjugants, which then divide. Its value is genetic recombination and rejuvenation. Clones that never undergo conjugation or autogamy lose vigour and die out after some 200 fissions (in P. tetraurelia).

(b) General features of egg-laying, pouched and aquatic mammals

Egg-laying mammals (Prototheria, order Monotremata: Ornithorhynchus, Tachyglossus, Zaglossus) live in Australia and New Guinea.

• They lay leathery-shelled eggs, which are incubated, and have a cloaca.
• Mammary glands have no teats; milk oozes from skin pores.
• Adults are toothless, with a duck-like bill (platypus) or a beak (echidna), and body temperature is low (platypus about 31 °C).
• The shoulder girdle keeps reptilian interclavicle and coracoid bones. The platypus has bill electroreceptors and a male venomous spur.

Pouched mammals (Metatheria, Marsupialia: Macropus, Didelphis, Phascolarctos): about 70% of species are Australasian and most of the rest South American.

• Gestation is very short (about 10–38 days) with a yolk-sac (choriovitelline) placenta.
• The newborn is tiny, blind and hairless; it crawls to a teat, usually inside the pouch (marsupium), attaches to it and completes development there. Some species have no pouch.
• Epipubic bones support the pouch. The female has two uteri and two lateral vaginae; the male usually has a forked penis.

Aquatic mammals (secondarily aquatic eutherians).

• Common features: streamlined body, thick blubber, flippers, reduced or absent external ear, large oxygen stores and a diving reflex.
• Cetacea (whales, dolphins, porpoises; Platanista, Balaenoptera): almost hairless, forelimbs are flippers, hindlimbs lost externally, horizontal tail flukes, a dorsal blowhole; toothed whales echolocate and baleen whales filter-feed.
• Sirenia (Dugong, manatees): herbivorous, flipper-like forelimbs, tail fluke, no hindlimbs.
• Pinnipedia (seals, sea lions, walrus): hindlimbs kept as flippers, nostrils at the snout, fur and blubber for insulation, and they breed on land.

(c) Types of scales in fishes (labelled diagrams)

Scales are dermal skin structures that protect the fish and reduce friction. The labelled diagrams should show:

• Placoid (sharks, rays): a rectangular basal plate in the dermis bearing a backward-pointing spine, with outer enameloid (vitrodentine), dentine and a central pulp cavity with blood vessels; they do not enlarge, and new ones are added as the fish grows.
• Cosmoid (extinct lobe-finned fishes, early lungfishes): thick rhomboid plate with outer vitrodentine, then cosmine (dentine-like), then spongy bone and basal lamellar bone (isopedine).
• Ganoid (Polypterus or bichir, gars, sturgeon): rhomboid, enamel-like plate with a glassy ganoine layer over dentine and bone, joined to its neighbours by peg-and-socket joints, forming armour.
• Cycloid (carps, Labeo, Catla): thin circular plate with a smooth margin, concentric growth rings (circuli) and radiating furrows, a bony layer over a fibrous layer, with the embedded and free parts marked.
• Ctenoid (perch, Tilapia): like cycloid but with comb-like teeth (ctenii) on the exposed posterior margin, which give a rough surface.

Cycloid and ctenoid scales are the thin, overlapping elasmoid scales of teleosts; cycloid scales are common on soft-rayed fishes and ctenoid scales on spiny-rayed, perch-like fishes. The evolutionary trend is from heavy cosmoid and ganoid plates to thin elasmoid scales, which make the body lighter and more flexible. Seasonal growth rings (annuli) on teleost scales are used to determine the age of a fish.