(a) Explain the general features, life cycle and pathogenicity of Fasciola. (20 marks) (b) Define Autogamy. Explain the mechanism…

(a) General Features, Life Cycle and Pathogenicity of Fasciola

Fasciola hepatica and F. gigantica are digenean trematodes, dorsoventrally flattened and leaf-shaped, acoelomate, with a syncytial, non-ciliated tegument bearing proteinaceous/scleroprotein spines or microtriches. They have a small oral sucker, a large subterminal ventral sucker, a blind digestive tract (mouth, pharynx, oesophagus and two intestinal caeca), protonephridia with flame cells, and hermaphroditic reproductive organs producing operculate eggs. F. gigantica, common in Indian ruminants, has larger eggs than F. hepatica; F. hepatica is more common in temperate regions, while F. gigantica predominates in tropical and subtropical India. Adults live in the bile ducts of cattle, buffalo, sheep, goats and humans, causing fascioliasis.

Life cycle. Eggs passed in faeces develop in water; the miracidium penetrates a lymnaeid snail (Lymnaea truncatula for F. hepatica; Lymnaea auricularia in India for F. gigantica), forming sporocysts and rediae, then free-swimming cercariae. Cercariae encyst on aquatic vegetation as metacercariae, the infective stage. The definitive host ingests metacercariae; they excyst in the duodenum, penetrate the intestinal wall, migrate through the peritoneal cavity and liver parenchyma, enter bile ducts, mature and reproduce.

Pathogenicity. Infection has an acute hepatic phase, caused by mechanical and toxic damage during parenchymal migration, and a chronic biliary phase, with bile-duct obstruction, cholangitis, hyperplasia, fibrosis, inflammation, anaemia, weight loss and reduced milk yield. In Indian livestock it causes major economic loss. Control combines triclabendazole deworming, pasture rotation, snail control and clean water.

(b) Autogamy in Paramecium: Mechanism and Significance

Autogamy is self-fertilisation within a single individual: the nuclear events of conjugation occur without a partner, and two gametic nuclei derived from the same micronucleus fuse. It was first described in Paramecium aurelia (Diller, 1936) and is distinct from binary fission. The two micronuclei undergo meiosis; after disintegration of the excess haploid micronuclei, a haploid micronucleus divides mitotically to form two pronuclei. Because no nuclear exchange occurs, the two pronuclei fuse to form a homozygous diploid synkaryon. The old macronucleus disintegrates, and the synkaryon divides twice to produce two new macronuclei and two micronuclei. The two micronuclei then divide mitotically before cytokinesis, so each daughter cell receives one macronucleus and the normal two micronuclei. The homozygous zygotic nucleus determines the genotype of the new macronucleus. The result is complete homozygosity, exposing recessive lethal alleles and producing genetically uniform clones. Its significance lies in survival when mates are absent, purging deleterious recessives, rejuvenating the clone by replacing the old macronucleus (resetting clonal ageing), and providing a model for ciliate nuclear genetics.

(c) Telencephalon in Vertebrates: Comparison

The vertebrate telencephalon is the forebrain region derived from the prosencephalon, but its size, evagination and pallial organization vary widely. In cyclostomes it is small, simple and only weakly evaginated, with little distinction between pallium and subpallium and a small corpus striatum. In cartilaginous fishes and lungfishes the paired hemispheres are evaginated, whereas in ray-finned (teleost) fishes the telencephalon develops by eversion: the pallium folds outward over a solid ventral mass, the ventricle becomes a T-shaped space covered by a thin membrane, and the subpallium (striatal area) lies ventrally. Amphibians show true evagination of the telencephalic hemispheres, with an enlarged corpus striatum and olfactory lobes, but no neopallium. Reptiles have larger, more evaginated hemispheres, an expanded pallium, and a well-developed corpus striatum. In reptiles, the pallium can be divided into archipallium, paleopallium and neopallium, but the neopallium is still less dominant than in mammals. In birds, the telencephalon is dominated by pallial structures—hyperpallium (Wulst), mesopallium, archipallium and the dorsal ventricular ridge—rather than by a hyper-developed corpus striatum; the DVR is pallial, not subpallial. The Wulst serves visual and somatosensory integration, and the DVR supports learning and multisensory integration. Mammals show the greatest development: large, highly evaginated cerebral hemispheres, a dominant neopallium/neocortex, reduced olfactory lobes, and a subpallial corpus striatum differentiated into caudate and putamen. The neocortex underlies higher sensory, motor and cognitive functions.

Thus, across vertebrates, telencephalic evolution is a progression from a small, subpallial-dominated organ to a large, pallial/neopallial organ, with corpus striatum modification and hemispheric expansion correlating with increasing sensory integration and behavioral complexity.