Seed Development in Citrus with Special Reference to 2X x 4X Crosses

Abstract

A B S T RAe T Comparative cytological and histological studies during embryogenesis of seeds from 2x X 2x and 2x X 4x crosses indicate that the ratio of ploidy level between embryo and endosperm is the most important factor affecting the course of seed development. The crosses produced seeds with the expected ploidy relationships between embryo, endosperm, and maternal tissue of 2:3:2 and 3:4:2 as well as the anomalous relationships 3:5:2, 4:6:2, and 6:10:2. All but 3:4:2 resulted in normal, germinable seeds. The ploidy level of the maternal tissue in relation to that of the embryo or endosperm did not appear to have any effect on seed development. About 92-99 % of seeds from 2x X 4x crosses containing triploid embryos with tetraploid endosperm aborted at different stages of embryogenesis. The abortion in all cases was preceded by abnormalities in the tetraploid endosperm. It is postulated that the unbalance of chromosome number between embryo and endosperm disturbs physiological relationships between these two tissues, leading first to the abortion of the endosperm and then of the embryo.

The utilities of Citrus tissue culture

“…The seedless trait in Citrus is related to male or female gametophyte sterility, self incompatibility, or early embryo abortion (Reforgiato Recupero et al 2005), and several methods exist for the production of seedless Citrus of which mutation breeding, somaclonal variation and triploid breeding are the most important. In Citrus, triploid seedless cultivars are obtained by breeding between elite monoembryonic diploid cultivars as female parent with tetraploid cultivars as pollen parent (Esen and Soost 1973). Sterility in such fruits is caused due to the odd number of chromosomes that are unable to undergo successful meiotic pairing to produce chromosomally balanced gametes (Reforgiato Recupero et al, 2005).…”

Breeding triploid plants: a review

“…Only the developed normal seed can germinate. Seeds tend to abort due to endosperm degeneration during early embryogenesis (Esen & Soost 1973;Sanford 1983). Esen and Soost (1973) proposed that the 3/4 ploidy ratio of embryo and endosperm caused the induction of seed abortion and endosperm degeneration.…”

Genetic Structure of Diploid Gametes for the Production of Triploid Citrus Hybrids

“…It is applied with two main objectives: (i) the development of triploid seedless cultivars mainly in the mandarin group, and to a lesser extend in the acid citrus group (limes, lemons) and (ii) the creation of new tetraploid rootstocks with a better adaptation to biotic and abiotic stresses. Spontaneous unreduced gametes have been widely exploited for triploid citrus breeding from diploid x diploid crosses (Esen and Soost, 1971;1973a;Aleza et al, 2010b; and the development of the tetraploid gene-pool also allowed to produce triploid hybrids by interploid crosses 2x x 4x and 4x x 2x (Esen and Soost, 1973b;Cameron and Burnett, 1978;Starrantino and Recupero, 1981;Grosser and Gmitter, 2011;Aleza et al, 2012a, b;. Most tetraploid parents are doubled-diploids, but also allotetraploid parents are used).…”

“…Citrus triploid plants can be obtained by 2x x 2x (Esen and Soost, 1971;1973a;Aleza et al, 2010b; or 2x x 4x and 4x x 2x sexual hybridizations (Esen and Soost, 1973b;Cameron and Burnett, 1978;Starrantino and Recupero, 1981;Grosser and Gmitter, 2011;Aleza et al, 2012a, b;. Sexual hybridization between diploid parents exploits natural events of polyploidization such as unreduced gametes, using embryo rescue and flow cytometry to select triploid hybrids (Ollitrault et al, 1996(Ollitrault et al, , 2007bAleza et al, 2010a,b).…”

“…Triploid citrus plants are currently being produced in various breeding programs for the development of new seedless commercial citrus varieties (Starrantino and Recupero, 1981;Grosser et al, 2010;. Triploid citrus plants can be recovered from interploid hybridizations, 2x x 4x and 4x x 2x (Esen and Soost, 1973b;Cameron and Burnett, 1978;Starrantino and Recupero, 1981;Grosser and Gmitter, 2011;Aleza et al, 2012a, b;, or by 2x x 2x sexual hybridizations as a consequence of unreduced (2n) gamete formation (Esen and Soost, 1971Soost, , 1973aAleza et al, 2010b;. The sexual 2x x 2x hybridization strategy was used by Geraci et al (1975) and to obtain triploid progenies using `Lisbon´ and `Eureka´ lemons as female parents.…”