1.  Introduction

 

The life cycle of angiosperms (higher plants) is characterized by alternating generations of sporophytes and gametophytes. The gametophytic phase arises when the diploid cells undergo meiosis (reduction division) to form male and female gametes. This phase is short lived as fertilization of the egg re-establishes the diploid sporophytic phase. The sporophytic phase characterized by chromosome number (2n) is the product of fertilization of male and female gametes, containing the haploid (n) set of chromosomes from each parent (Forster et al., 2007). Therefore, haploid is a generalized term for plants that contain the gametic chromosome number (n). This is in contrast to diploid plants, which contain two sets (2n) of chromosomes. Haploids are sexually sterile and, therefore, doubling of the chromosomes is required to produce fertile plants which are called doubled haploids or homozygous diploids.

Haploids have attracted great interest of plant physiologists, embryologists, geneticists and breeders. Spontaneous production of haploids usually occurs through the process of parthenogenesis (embryo development from an unfertilized egg). However, occasionally, they bear the characters of male parent only, suggesting their origin through ‘ovule androgenesis' (embryo development inside the ovule by the activity of the male nucleus alone where elimination or inactivation of egg nucleus occurs before fertilization). Although in vivo occurrence of haploids has been reported in several species but at low and variable frequencies and was regarded as a special biological phenomenon (Figure 9.1). The low frequency of spontaneously arising these haploid plants severely limited the utilization of haploids for crop improvement and genetic studies.

Figure 9.1: Twin embryos excised from a Neem seed. Smaller embryos (arrow marked) is a synergid embryo formed without fertilization and is haploid. The bigger embryo is zygotic embryo which is formed during fertilization of egg cell with the male gamete

In vitro Haploid plants can be obtained by triggering the male or female gametic cells to undergo sporophytic development. In vitro androgenesis (anther-microspore culture) is one of the most preferred techniques for obtaining haploids but, in vitro gynogenesis (unfertilized ovary-ovule culture) can prove to be a complementary technique in species where anther culture is inaccessible or less productive. It means that not only the microspore but, also the megaspore of angiosperms can be triggered in vitro to undergo sporophytic development. However, production of haploids via gynogenesis is more tedious, less efficient in comparison to androgenesis. In addition to the above techniques, in situ parthenogenesis (pollen irradiation and chemical treatment) can be employed for generation of haploid plants (Kurtar and Balkaya., 2010).

Haploid and diploid lines play a vital role in genomics and have been used for the purpose of physical mapping, genetic mapping and also for integration of genetic and physical maps. Additionally, haploid and doubled haploid plants are adapted for mutagenesis and genetic transformation experiments, presenting the advantage of immediate production of homozygous lines. It is also expected that, in the near future, haploid and doubled haploid plants will play an increasingly important role in whole genome sequencing projects, where homozygosity is of particular advantage. The technique has special significance for genetic improvement of the tree species where breeding is made difficult due to long generation intervals and highly heterozygous nature of these plants as a result of cross pollination.