Speciation pdf

Analysis of these factors will help to get a detailed insight about the process of gradual progression of the concept of speciation. At end, the report also aims to highlight the future implication of the study in the domain of speciation.

In order to conduct this research I employed a thematic analysis approach based on the search of the literary articles. The articles were searched in the electronic databases like Medline and PubMed. On the basis of the keyword search I found few relevant articles which I will use in the body of the assignment to backup my arguments.

These references will be helpful because it will help to draw a critical insight about the factors contributing towards speciation with insightful examples. Emergence of Reproductive Isolation and Speciation As per the biological species concept, understanding the concept of speciation demands proper insight about the process of breakdown of gene flow Butlin et al. The process of gene flow can be critically analysed upon discussion of the reproductive isolation.

Butlin et al. Thus, it can be regarded as an important process behind the concept of speciation. The view stated by Butlin et al. According to Sobel et al. This is because an organism is required to mate outside its own herd before it generates a hybrid yet sterile progeny. In the domain of reproductive isolation, Riesch et al. According to the theoretical studies, the culture can be regarded as important evolutionary mechanisms due the ability of the cultural traits to spread rigorously in both vertical, horizontal and oblique direction Schluter, This results in decrease in variance within the group and increase in variance between the groups.

In their study Riesch et al. They have argued that they are in a process of ongoing ecological speciation arising out of dietary specialization. For example, hexaploids derived from A.

Garcia et al. Germplasm lines have been released from this cross with resistance to early leaf spot, nematodes, and sev- eral insect pests Isleib et al.

A second method to introgress germplasm from diploid species to A. This method has the advantage of avoiding several generations of mostly sterile hybrids and recovering tetraploids is much faster than by going through the triploid—hexaploid procedure; however, autotetraploids generally have low vigor, and when annual species are used as parents, they are short-lived.

TxAG-6 had very good nematode resistance, but also significant linkage drag which resulted in low yields and poor seed and pod quality. RFLP markers linked to the resistance nematode gene were used to select favorable genotypes Church et al. By using SSR markers, Nagy et al.

Alleles from wild species show a clear potential for peanut improvement. However, the use of wild species in peanut breeding has been hindered by the ploidy differences between the cultivated and wild species, linkage drag of unde- sirable wild alleles with desirable ones, and difficulties of tracking introgressed fragments from wild genomes.

Some wild introgression have been incorporated into commercial cultivars in the USA: strong resistance to root-knot nematode Holbrook et al. A number of rust and late leaf spot resistant cultivars have been developed from a line known as GPBD 4 which in turn has a parent ICGV that is derived from an A.

These exam- ples show the potential for the use of wild species in cultivar development. Molecular markers offer a method for following introgression from Arachis species to A. Different DNA marker systems can be used to monitor chromosome segments.

Microsatellite or SSR markers have become the assay of choice for genetic studies in Arachis because they are multiallelic, codominant, transferable among related species, poly- merase chain reaction-based markers, and usable in tetraploid genomes.

Efforts by several research groups to develop microsatellite markers for peanut have resulted in more than 15, SSRs Pandey et al.

These markers have been used to study diversity in the genus Arachis Barkley et al. Single nucleotide polymorphism SNP markers constitute the most abun- dant molecular markers in the genome and can be carried out with high through- put genotyping methods. SNP markers have been widely used in many plant species. However, they have had limited use in peanut because separation of A and B genome sequences is required. Using the same assay, Bertioli et al. A large set of SNP data is currently available and is being mined from tetraploid 26 diverse genotypes and diploid four species genome and transcriptome sequences.

To date, the number of genes associated with molecular markers in peanut is relatively small, but the large number of molecular markers becoming avail- able has great potential for utilization in crop improvement programs. Bertioli et al. Pandey et al. Chu et al. A few examples will be presented in the following paragraphs. Root-Knot Nematode Meloidogyne spp.

The first markers for an agronomically useful, wild species-derived trait in peanut were for resistance to RKN M. Two closely linked sequence characterized amplified region SCAR markers were identified for genes for reduced galling and egg number Garcia et al. Simultaneously, three RAPD markers were associated with nematode resis- tance in several backcross breeding populations derived from the interspecific hybrid TxAG-6 A. Marker-assisted selection then was used to develop several high yielding, nematode-resistant cultivars.

In this case, it was demonstrated that use of markers was more efficient than phenotypic selection because plants selected with markers for the homozygous resistance gene have fewer escapes compared to plants from phenotypic selection. Burow et al. In spite of the success of this work, it is now thought that the use of a single gene trait that confers near immunity may be subject to breakdown of resis- tance under high selection pressure and is cause for concern. Therefore, new sources of resistance to nematodes are needed.

The A genome species A. Induced allotetraploids were produced using A. It is expected that introgression of these alleles into peanut will give rise to nematode-resistant cultivars.

Late Leaf Spot Resistance Resistance to LLS Cercosporidium personatum has multiple components, including percent defoliation, incubation period, latency period, lesion num- ber and diameter, sporulation, and pod yield Anderson et al.

High levels of resistance also have been associated with low yield which suggests link- age or pleiotropic effects Iroume and Knauft, , thus breeding for high yielding cultivars with resistance requires that this linkage be broken. Stalker and Mozingo identified three RAPD markers associated with early leaf spot lesion diameter in a peanut population derived from a cross between an A.

Those QTLs for latency period and lesion number were overlapping, suggesting either linkage between the two or a QTL with pleiotropic effects. Additionally, Leal-Bertioli et al. The results suggested additive or partial domi- nance gene action. One QTL explained almost half of the phenotypic variance observed and some QTLs mapped near resistance gene analogues-based mark- ers.

GPBD 4 was a common parent in all of these crosses, and since it was derived from the wild species A. Groundnut Rosette Virus The aphid-transmitted groundnut rosette virus is an important pathogen of pea- nut in Africa and Asia, causing severe stunting and loss of yield. Herselman et al. A segregating population of F2 plants of the A genome diploid cross A. In Brazil, interspecific populations and wild species also have been found as promising for introgression of resistance to the thrips, Enneothrips flavens Janini et al.

It was shown that wild alleles contributed positive variation to several agronomic traits such as flowering precocity; seed and pod number per plant; and length, size and maturity of pods. Moreover, the comparison of QTLs obtained under well-watered and water-limited conditions revealed that QTLs for stress tolerance indices for pod and seed numbers with favorable alleles could be attributed to the wild parents.

These could be involved in reproductive trade-offs between producing large seeds versus producing more, but smaller, seeds under water stress. Large col- lections of cultivated peanut exist at multiple locations and several hundreds of wild species are maintained in germplasm banks.

Many of the species have been characterized for agronomic traits, but much of the germplasm collection remains to be evaluated for disease and insect resistances. The incorporation of wild alleles into crops is a proven strategy to develop improved varieties with pest and disease resistance.

However, the extent of utilization of the use- ful allele reservoir in wild species and its impact on peanut breeding has been relatively limited because of restrictions to crossability, multiplication rate, and, until recently, to the lack of appropriate molecular tools to analyze and follow traits in hybrids. Arachis cardenasii has been one of the most useful sources of genes from wild species to date, but crosses involving other species are being produced, in particular A. The recent use of the two most probable ancestors of peanut A.

As new materials are being created and genotyping strategies are becoming more advanced, variability from the wild species is being harnessed to the benefit of world agriculture. Structural differences in stigmas of Arachis species Leguminosae and their probable significance in pollination.

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