0000018324 00000 n This course provides the basic biology you need to understand all of these issues better, tries to clarify some misconceptions, and tries to prepare students for future, more advanced coursework in Biology (and especially evolutionary genetics). 0000009168 00000 n c) Hybrid inviability. Chapter 24 21 Terms. 0000009851 00000 n To those biologists weary of the intricacies of this debate, stabilization theory offers a clear, brief explanation of the general phenomenon of hybrid infertility: Populations treated as distinct species often belong to distinct chromosets. I cannot (in. 0000016395 00000 n An example of hybrid sterility: when a horse and donkey are crossed, a mule is born. trailer << /Size 99 /Info 42 0 R /Root 44 0 R /Prev 132670 /ID[<05f68074c22668e6ca298658937c8428><05f68074c22668e6ca298658937c8428>] >> startxref 0 %%EOF 44 0 obj << /Type /Catalog /Pages 41 0 R /PageLabels 40 0 R /Outlines 38 0 R >> endobj 97 0 obj << /S 366 /O 511 /L 527 /Filter /FlateDecode /Length 98 0 R >> stream 2. a postzygotic barrier to reproduction. STUDY GUIDE. ;����'�sw��D����#��t��-A!8��mTܳ��J�-��D�1�p�W;��k��+h�&8���T���h�"T h?H�)-&z��z�/tg/IB[�"A# I��X�)�qK���o��`E�AS����yZ>�iF-!$���b1�2��y糇�!� ��l� Mechanical differences. It is because the karyotype of each population comes into being independently of the karyotypes of other populations. Being like other members of the population pays a large reproductive dividend (any immigrant or mutant individual with an aberrant karyotype will not find a mate of its own kind and so will produce structurally heterozygous progeny of low fertility). Various theories have been offered to explain why hybrids are commonly less viable and/or fertile than their parents. Browse. Horses and donkeys remain separate species because of this hybrid sterility, which is: 1. an allopatric barrier to reproduction. Hybrid breakdown is when the hybrid develops into a healthy offspring that carries the capability to reproduce. 0000014698 00000 n 2014). Really, this seems to be little more than a vague, old idea that even predates any precise science of genetics. 0000019286 00000 n 0000003318 00000 n Therefore, their fertility is expected to be reduced. In evolution: Hybrid breakdown In plants more than in animals, hybrids between closely related species are sometimes partially fertile. 0000007262 00000 n Hybrid rice, which was adopted in the 1970s, now covers nearly 13.6 million hectares each year in China alone. It is likely that genetic bases of hybrid sterility are extremely diverged in differ-ent systems. Sperm transfer takes place, but egg is not fertilized. 0000001467 00000 n The offspring of the interspecific crosses are mainly sterile. In this scenario, two distinct somasets are derived from a single ancestral somaset by divergence in geographic isolation. He also refers to this mystery factor in The Descent of Man (1871: vol. Hybrid sterility is the most common form of postzygotic reproductive isolation in plants. (1999); Gustavsson (1990); ; King (1980); King (1993: 164â168); Lewis (1966); McCarthy (2006); McClintock (1945); McCarthy et al. The best-known example is perhaps the hybrid sterility between indica and japonica subspecies of Asian cultivated rice (Oryza sativa L.). 0000017286 00000 n 0000019517 00000 n But there is even a higher grade of sterility than this. Hybrid Sterility: Even if the hybrid offspring is otherwise perfectly healthy, either one or both sexes is sterile. Hybrid male sterility (HMS) is a form of postmating postzygotic isolation among closely related species that can act as an effective barrier to gene flow. Under such circumstances hybrids are expected to be structural heterozygotes, which are typically less fertile than their homozygous parents. Which of the following is not an example of the evolution of a key innovation (a new trait) allowing occupation of a new adaptive zone (habitat or lifestyle). This sounds well enough. Abstract. 0000008538 00000 n It is the hybrid offspring between a male donkey and a female horse. that hybrid sterility is a complex phenotype and is con-trolled by complicated mechanisms involving multiple loci. Thus, it is said, reproductive isolation eventually becomes complete, as hybrids become less viable and fertile. 0000012425 00000 n Indeed, no explanation, accounting in terms of natural selection, for the general finding that hybrids are typically of reduced fertility is accepted by all biologists. 0000023301 00000 n The exploitation of male sterility systems has enabled the commercialization of heterosis in rice, with greatly increased yield and total production of this major staple food crop. Hybrid male sterility (HMS) contributes to reproductive isolation commonly observed among house mouse ( Mus musculus ) subspecies, both in the wild and in laboratory crosses. After all, if any two individuals are not a pair of clones, they will differ genetically. 43 0 obj << /Linearized 1 /O 45 /H [ 1560 549 ] /L 133658 /E 72397 /N 10 /T 132680 >> endobj xref 43 56 0000000016 00000 n However, there is really no reason for us to expect such hybrids to be sterile. Chromosomal rearrangements between the subgenomes of a hybrid can also lead to sterility due to aberrant chromosome pairing and segregation (8, … Understanding the nature and number of genetic changes that underlie early events in speciation remains a major challenge in evolutionary biology.Experiments to identify “speciation genes” have become popular and often focus on loci affecting hybrid male sterility, an attractive trait because it is considered “discrete” and thus easy to score. However, within a second chromoset there can be selection for uniformity with respect to some other karyotype. ... in genera of plants including numerous species, a series can be formed from species which, when crossed, yield fewer and fewer seeds, to species which never produce a single seed, but yet are affected by the pollen of the other species, as shewn by the swelling of the germen. 2014). Discoveries in such diverse disciplines of science as radiation biology, chemistry, animal behavior, molecular biology, genetics and entomology have contributed to our current ideas concerning the control of insect pests. So there is nothing to coordinate the structures of the two resulting karyotypes, and they will likely differ in some, or many, respects. The hybrid sterile organism can be healthy and fit, but it is unable to produce offspring. 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