Editorial: Doubled Haploidy in Model and Recalcitrant Species

Doubled haploid (DH) technology is a powerful tool in plant breeding to reduce the time and costs needed to produce pure lines, the cornerstone of hybrid seed production. This biotechnological alternative to classic methods allows for a reduction of the typical 7–8 inbreeding generations needed to fix a hybrid genotype to only one in vitro generation. It is therefore much faster and cheaper, being the principal advantage of DH technology in plant breeding, but not the only. Indeed, DHs are also useful for genetic mapping of complex qualitative traits, for linkage studies and estimation of recombination fractions, to unmask recessive mutants, to avoid transgenic hemizygotes, or for reverse breeding, among others (Forster et al., 2007; Dunwell, 2010; Dwivedi et al., 2015). These are some of the advantages that make DH technology one of the most exciting fields of present and future plant biotechnology. At present, there are several ways to produce haploids and eventually DHs (after a process of chromosome doubling), involving both female and male gametophytes. From the female gametophyte, haploids may be produced by uniparental genome elimination and by induction of gynogenesis. Uniparental genome elimination is typically achieved by crossing two sexually incompatible species, in some intraspecific crosses when one genitor carries specific mutation(s), or through genetic manipulation of CENH3, a centromeric variant of the H3 histone (Ravi and Chan, 2010, 2013; Karimi-Ashtiyani et al., 2015). Gynogenesis is a route through which unfertilized ovules, ovaries or even entire flowers are cultured in vitro to induce the development of a haploid embryo, generally from the egg cell (Bohanec, 2009). From the male gametophyte, haploids may be obtained through androgenesis (Seguí-Simarro, 2010). The most common and useful androgenic pathway is microspore/pollen embryogenesis, through which microspores/pollen are reprogrammed toward embryogenesis. Discovered more than 40 years ago (Guha and Maheshwari, 1964), this process has become of great practical importance for the agronomic industry due to its convenience for producing DH lines much faster, cheaper, and in more species than the other methods above mentioned (Forster et al., 2007; Dunwell, 2010). This is why when possible, microspore embryogenesis is the method of choice to produce DHs. For all these methods, there are species where they are most efficient. This is why these species are used as experimental models to study basic aspects of the process. This is the case of onion for gynogenesis, and of barley and rapeseed for microspore embryogenesis in monocots …