Production of doubled haploids in crop plants. An introduction

Abstract

Haploid is the general term for plants (sporophytes) that contain the gametic chromosome number (n). Thus, in a diploid sporophytic (2n) species the haploid could also be called monoploid (x) as they have only one set of chromosomes. In polyploid species, the haploids (n) have more than one set of chromosomes and are polyhaploids. The haploid plant from an autotetraploid (4x) with four sets of one genome was originally called a dihaploid (because 2n=2x). It is important to note that when the chromosome number of a haploid is doubled, it should be called a doubled haploid (DH) and not a dihaploid. The dihaploid is not homozygous as it represents two chromosome sets selected from four sets in the autotetraploid, whereas the doubled haploid from a monoploid or an allohaploid should be completely homozygous. Since there has always been concern about the terminology used in haploidy, R. A. de Fossard was asked to formulate guidelines on terminology to be presented at the 1st International Symposium on Haploids in Higher Plants in 1974 (Kasha, 1974). A group of scientists met to discuss this terminology guideline and the consensus was adopted by the Symposium and published in its Proceedings (de Fossard, 1974). Thus “x” or multiples of “x” should be used to denote the number of sets of chromosomes while “n” and “2n” are used for the gametic and sporophytic chromosome numbers respectively. For example, a bread wheat plant (Triticum aestivum L.) could be described as 2n=6x. Its usual haploid gamete would be n=3x and the sporophytic plant from the gamete would be 2n=3x. In addition to euhaploids with complete sets of chromosomes, one could also produce or find aneuhaploid plants where the chromosomes do not represent balanced complete sets of chromosomes (e.g. 2n=3x−1 or 2n=3x+1). For the many other possible variations please see de Fossard (1974). Other valuable terms are androgenetic haploids that are derived from the male gamete and gynogenetic haploids that are derived from the female gamete.