Breeding plantation tree crops. Tropical species

Abstract

This Tropical species volume of the Breeding plantation tree crops series, edited by S. Mohan Jain and P.M. Priyadarshan, is aimed at providing ‘comprehensive information on a package of conventional breeding, biotechnology and molecular tools to scientists, students and even policy makers and planters’. The volume has a total of 16 chapters on fruits and nuts (banana, mango, guava, papaya, grape, date palm, litchi, avocado and cashew), oil crops (coconut, oil palm and olive), industrial crops (rubber) and beverages (coffee, tea and cocoa). Each chapter is devoted to one single crop and follows roughly the same shape: economic importance, botany, genetic resources, cultivars, breeding techniques, breeding progress and biotechnology. The chapters are written by recognized international specialists in their respective fields of research. A colour insert of 20 pages is provided in order to illustrate genetic diversity, floral biology and hybridization techniques for several species. Unfortunately there are several cases of unnecessary duplication of black and white/colour pictures in the text and colour insert, and in many cases one single picture giving an overview of genetic diversity would suffice (see Figs 10.2, 16.4). Clearly the originality of this book resides in the integration of both breeding and biotechnology data on species of important commercial value. There are a few recent books on biotechnology applied to this type of plant species, such as Biotechnology of fruit and nut crops edited by Litz (2005), although the subject of breeding tropical perennial crops has not been recently reviewed to my knowledge. Perennial crops demand huge areas and labour-intensive breeding on long selection cycles; they require large populations for inheritance studies. As an example, Batugal et al. (Chapter 10) note that 8 hectares are needed for a period of 12 years for a standard genetic trial for coconut. High investments of financial and human resources under a secure environment are needed over a long period, which directly impacts upon breeding strategies and international partnership. Such long time frames of conventional breeding make biotechnology very attractive in some cases (e.g. mango, date palm and oil palm). In many cases, such as cashew breeding, high heterozygosity coupled with an allogamous character demands large samples to represent natural variability (see de Paiva et al., Chapter 9). The floral biology of the species directly impacts on breeding strategies and the need for biotechnology. For example, early molecular markers of sex differentiation are of paramount importance for date palm breeding (El Hadrami, Chapter 6). Mango cultivars (Bally et al., Chapter 2) can be mono- or polyembryonic: in this case one (weak) embryo is zygotic and the rest are nucellar in origin, and they are similar to the tree bearing the seed. The use of polyembryonic parents for breeding purposes is difficult, as only molecular tools are able to discriminate zygotic from nucellar embryos. An original characteristic of papaya (Chan, Chapter 4) is the existence of five basic types of flowers, which can undergo sex reversal and morphological changes under the influence of environmental changes. This original characteristic governs genetic studies as the sex of papaya is determined by monogenic inheritance. The genetic basis of modern planting material is sometimes as narrow as a handful of imported (or smuggled!) seeds (oil palm, rubber, coffee), or else it might derive from an important natural variability (cocoa). For banana, Bakri et al. (Chapter 1) report that the genetic basis is very narrow and the crop is at risk because of the emergence of new diseases. Several authors show that the dispersion and domestication of cultivated plantation crops followed the routes of local trade, then those of colonial exploration (rubber, coconut, oil palm, cocoa, mango, tea), and more recently the flows of migrants (olive). For several of the crop species under review in this book, the history of evolution and domestication of species involves the generation of aneuploid, tetraploid or triploid varieties (banana, coffee). These characteristics make the breeding process and the introduction of biotechnology even more difficult. Several of the authors make a valuable effort to provide a worldwide review of the state of the art in genetic resources management and international collaborative breeding initiatives (Bakri et al., Chapter 1; Batugal et al., Chapter 10; Mondal, Chapter 15; Monteiro et al., Chapter 16). This is not always the case, however, and some chapters are too focused on results obtained within the authors' institution (de Paiva et al., Chapter 9; Sambanthamurthi, Chapter 11), which limits the interest of the review. The role played by global networks managed by international bodies is underlined for several crops (banana, coconut, cocoa). International networking is involved in key steps of breeding strategies, such as databank management, multiloci trials, management and exchanges of germplasm, or the development of new technologies like genomics. Genetic improvement still relies on traditional breeding techniques in all the species reviewed. Biotechnological approaches such as MAS (marker-assisted selection) are more-or-less integrated into breeding strategies, and this rate of incorporation ranges from low (mango) to high (oil palm). It appears that MAS and micropropagation are the most developed techniques for many species. The in vitro culture of zygotic embryo is routinely used for the safe movement of germplasm (coconut) and the safeguard of genetic resources (olive). Various different techniques based on PCR have been developed on guava (Pommer and Murakami, Chapter 3) in order to fingerprint accessions, aid genetic diversity studies, construct genetic linkage maps, and tag genes of interest for MAS. In several cases, high-throughput fingerprinting is underway (Batugal et al., Chapter 10; Fabbri et al., Chapter 12). International networks are needed to concentrate efforts in genomics, like the ICGN for coffee. Successful introgression of useful traits through genetic engineering remains rare, although commercial varieties of papaya (Chan, Chapter 4) now include F1 hybrids obtained from a cross between commercial cultivars and transgenic varieties with coat-protein-mediated virus resistance. Sambanthamurthi et al. (Chapter 11) report on the successful generation of transformed oil palm plants. In conclusion, I would highly recommend this book to scientists (both breeders and plant biotechnologists), students, agronomists and … tropical farmers too! It provides an updated vision on breeding and biotechnology applied to crop species of paramount economic interest for the tropical world.