Читать книгу Cucurbits - James R. Myers - Страница 40

The nature of DNA in cucurbits has been under study for more than three decades. Bhave et al. (1986) analysed the distribution of repeat and single copy DNA sequences in smooth and angled luffa, wax gourd and ivy gourd. Around the same time, Ganal and Hemleben (1986) compared restriction enzyme maps of ribosomal DNA repeats in cucumber, melon, and two squash species, C. maxima and C. pepo. An early and continuing objective for studying DNA and RNA in cucurbits has been to clarify phylogenetic relationships within the family. Analysis of variation of chloroplast DNA has revealed species relationships in Cucurbita (Wilson et al., 1992) and Cucumis (Perl-Treves and Galun, 1985), and researchers continue to use DNA-based information systems to detect origins and evaluate relationships in the Cucurbitaceae (Schaefer et al., 2009; Chomicki and Renner, 2014).

Speed breeding is a system that makes use of new technology to develop cultivars in less time. Some of those technologies that have been applied to speed breeding of cucurbits are as follows. Greenhouses can be supplied with lights that permit faster growth and flowering in the winter season. In addition, temperature control permits rapid growth, usually with the objective of 32°C day and 21°C night. Field trials can be run faster by using many locations and few years. Speed breeding usually builds on previous technologies. Some of those include winter nurseries, optimum size of roots in greenhouse pots or bags, and the use of growth regulators to increase the speed of making crosses and self-pollinations.

Much of the early progress made towards understanding the genetic code of cucurbits and other plants was due to the use of restriction enzymes, also called endonucleases. An endonuclease cleaves DNA at a particular location based on the enzyme’s recognition of a certain sequence of nucleotides. There are many different endonucleases, each cutting DNA at different places, and creating DNA fragments of various lengths. Endonucleases allowed for DNA mapping, which has been going on for cucumber, melon and squash for the past 30 years. For example, Gounaris et al. (1990) used endonucleases to resolve the genetic map of chloroplast DNA in C. pepo, and more recently, endonucleases were used to generate a genetic map of zucchini (C. pepo) using genotyping by sequencing (GBS) (Montero-Pau et al., 2017).

Endonucleases also enable scientists to create recombinant DNA, which results when cleaved DNA strands from two different DNA molecules are enzymatically joined. If one of these molecules is a bacterial plasmid (an independent, self-replicating ring of DNA in the bacterial cell), then DNA from the other molecule can be incorporated into the plasmid. The plasmid containing the recombinant DNA can be transferred to embryonic plant cells, which, in turn, pass the genetic material to new cells.

When the transgenic vector is a bacteriophage (a bacteria-infecting virus) instead of a plasmid, the recombinant DNA is passed from virus to bacterium (where it is incorporated into the bacterial DNA), and then from bacterium to plant. Bacteria containing incorporated viral DNA possess a degree of immunity to attacks by the same phage because the incorporated DNA directs synthesis of a phage repressor molecule that regulates the expression of the viral DNA.