Question

Question 8 4 pts Many organisms are hermaphroditic and make both eggs and sperm. If a single individual self-fertilized (contributed both the egg and sperm), would the resulting offspring be genetically identical to the parent? If not what contributes to the diversity? Explain your answer. B I VA A IX E 31 3XX, DE - ? VOGT 12pt Paragraph I O words

Answer 1

Hermaphroditism can be seen in many of organisms.while the method of self fertilization causes inbreeding depression in plants in many animals it is avoided by a different procedure:

*1 - Simultaneous hermaphrodites that self fertilize : self fertilization in vertebrates is rare but one known example is Rivulus marmoratus a small killifish of neotropical mangrove swamps . Eggs are fertilized internally  with sperm from the fish's own testis, and the eggs are guarded after spawning. In these gonads, mature eggs and sperm are “ovulated” into a common lumen, where fertilization occurs. Note, however, that if the eggs are exposed to temperatures below 19°C, two thirds of the young develop as primary males that will never develop ovarian tissue. These males will mate with hermaphrodites, thus reducing the tendency to homozygous clones. In addition, if young hermaphrodites are exposed to high temperatures, they develop as secondary males that can also mate with hermaphrodites.

*2 Sperm competition- In some organism such as Caenorhabiditis elegans mated hermaphrodites produce predominantly outcrossed progeny because of the competitive superiority of male-derived sperm (termed “sperm competition”). This phenomenon is a primary reason that C. elegans is such a powerful genetic system. The transition towards outcross progeny typically occurs within 1 day after male-introduction; by day two, progeny are almost exclusively outcrossed .

Sperm competition is independent of the ability to fertilize; the sperm of fertilization-defective C. elegans mutants can effectively outcompete hermaphrodite sperm, reducing the self-fertility of these mated hermaphrodites . The dominance of male-derived sperm may be partially due to size; C. elegans male-derived sperm are approximately 50% larger than hermaphrodite-derived sperm, move faster, and are thought to displace them from the distal end of the spermatheca . However, size per se cannot be the complete explanation, as C. remanei male-derived sperm do not reduce self-fertility in mated C. elegans hermaphrodites, despite being two times larger in diameter . C. elegans hermaphrodites may also have an independent mechanism for actively selecting functional sperm .

Sequential mating experiments with multiple males suggest sperm competition in C. elegans is limited to males versus hermaphrodites. This is perhaps not surprising given that in wild populations male sperm are more likely to encounter hermaphrodite sperm within the spermatheca than sperm from another male .

The competitive ability of male-derived sperm has useful implications for the study of sterile mutants in C. elegans. To be competitive, the males themselves must successfully mate and transfer sperm to the hermaphrodite while their sperm must have successfully completed in vivo spermiogenesis, become motile, and migrated to the spermatheca. Sperm competition assays are therefore valuable tools for the characterization of sterile mutations in C. elegans, but these assumptions should be confirmed via other direct means . In a typical experiment, morphologically marked L4 hermaphrodites (e.g. dpy-5) are crossed with unmarked mutant males. The numbers of outcrossed (Non-Dpy) and selfed (Dpy) progeny are counted, and the percentage self-progeny is recorded.

In addition to unmated hermaphrodite and wild-type male controls, only healthy and age-matched worms should be used as male mating behavior and efficiency may be affected by age and overall health. To minimize variations in the microenvironment, all crosses should be done concurrently using the same food batch and incubator.

*3 Comparitive reproduction -

Though some clitellates are capable of self-fertilization (e.g., Tubifex tubifex) or parthenogenesis, most cross fertilize by copulation. This involves physical contact between two individuals and the mutual exchange of sperm, usually via an intromittent organ or by the transfer of a sperm-filled sac called a spermatophore. Spermatophores are cemented to each other in the prostate bulb and transferred to the ventral body surface of the partner, typically in the clitellar region. Sperm exit the spermatophore, and by a mechanism not fully understood find their way via the coelomic sinuses to ovaries, where the eggs are fertilized internally.

Spermatogenesis occurs in paired testisacs and resulting spermatozoa pass through structures including the vas deferens and atrium before longer term storage in the epididymis (Fig. 4). Paired ovisacs give rise to oogonia and oocytes, which pass through oviducts that converge and lead to the female pore. Eggs may acquire yolk by a process called vitellogenesis and can be quite large (up to several mm in some Glossiphoniids), or yolk may be deposited into cocoon fluid as a nutritional source for developing embryos (albumenotrophy). Sperm from a partner may be stored in specialized compartments called spermathecae, and released during egg laying to fertilize eggs internally or in the cocoon.

Fig. 4. Dorsal view of a clitellate (Helobdella sp.) reproductive system (A) Position of reproductive system in body. (B) Dorsal dissection. (C) Schematic reproductive organs. a, atrium; ac, atrial cornua; os, ovisac; od, oviduct; sd, sperm duct; t, testisac; vd, deferens.

*4 Outbreeding -

A parent contributes both genomic complements to offspring derived by self-fertilization, but only one to offspring derived by outbreeding. Under random fusion of gametes, the gamete contributed by the other parent is randomly sampled from the population. However, the expression of mating incompatibilities ensures that the gamete from the other parent is less similar than random in regions that cosegregate with the mating type locus. Mutations within the mating type locus region that suppress mating incompatibilities would appear to enjoy a greater than twofold advantage over functional mating type alleles.

In spite of this enormous disadvantage, mating incompatibilities have persisted over long periods in a variety of organisms. Suppression of mating incompatibilities may induce severe inbreeding depression. Enforced heterozygosity of mating type alleles can shelter recessive deleterious mutations from expression and purging, permitting their accumulation in regions closely linked to mating type loci. Upon the suppression of mating incompatibilities, such mutations would be expressed in offspring carrying mating type alleles in homozygous form. The progressive accumulation of such deleterious factors may constitute a strong and intensifying force serving to maintain outbreeding.

Above mentioned processes maintain the diversity among hermphrodatic organisms . Hence there will be offsprings which are similar to parents by inbreeding and genetically diffrentvones are prodoced through Outbreeding.

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