Asparagus breeding - problem solving

Asparagus being dioecious, sex determination is of the XX, XY type. However, no heteromorphic X or Y chromosomes can be detected, so sex may be determined by a single locus. The karyotype consists of 6 large and 4 small pairs of chromosomes.

Symbols used for sex genotypes:

XY male (Mm)

XX female (mm)

YY supermale (MM)

Significance of andromonoecy:

Approximately 1/1000 of XY males are andromonoecious (plants with male and perfect flowers). The perfect flowers of these plants develop small fruits with viable seeds, which will produce progeny segregating 3:1 [male (2XY: 1YY), to female (XX)]. The YY (supermales) indistinguishable from the XY males.

Andromonecious males are selected on the basis of berry number.

They should not produce more than 10 per year, otherwise male hybrids will have too many berries thus having the same problem of the females defeating the purpose. That is, female plants tend to be lower yielders, because they spend more energy than males in berry formation. If the berries are removed in the female plants, yields increases 10-20%.

YY supermales can be used to cross with XX for producing all male hybrid varieties as explained below.

Model for control of andromonecious phenotype:

A model has been proposed to explain the genetic basis of this trait, based on two linked loci:

1) Dominant suppressor of female flowers (S)

2) Dominant activator of male flowers (A)

Both genes are closely linked.

Male SsAa

Female ssaa

Rare recombinants, Ssaa, ssAA results in andromonoecious.

Environment also plays a role in the expression of andromonoecy. For example, at the end of the season, males tend to produce a few perfect flowers.

The same structural genes are present in male and female flowers, but different regulatory genes controlling the appearance of reproductive organs.

The pathway for flower development is remarkably similar in both sexes up to meiosis. Until this stage, male and female flowers contain primordia for stamens and pistils. Soon thereafter, the stamens degenerate and pistils develop into female flowers. In male plants, pistils stop growing but do not degenerate, and stamens continue to develop. Sex specific mRNA has been isolated. Also specific protein spots have been identified as sex specific. It seems to be that the absence of translational control in the male rudimentary ovaries causes of inability to proceed with normal ovary development.

Polyembryony:

Occurs in about 1% to 3.5% of the seeds, producing mostly twins, seedlings, although triplets and quadruplets also occur.

These are a source of haploids, triploids and even trisomics. The frequency of haploids is 18/1000 polyembryonic seeds.