Use of genetic markers at nursery stage to select good varieties

By R. Bourdeix, 2018

Some coconut varieties can usefully be identified at the nursery stage by looking at the color of the sprout of germinating seedlings. This color genetic marker, well known by many farmers for instance in India, is of crucial importance because it allows to differentiate between varieties before planting them.
The most common use by farmers is to recognize, within the progenies of red and yellow dwarfs, the pure red and yellow dwarf seedlings from the few natural Dwarf x Tall hybrids. The main color segregation is genetically based on a simple determinism involving two couples of independent alleles (Bourdeix, 1988). 
The varietal mix illustrated in Figure 1 was planted in two isolated small islands of Samoa. For instance, if a green variety is planted together with red or orange varieties, as shown in Figure 1, the color of the sprout will be used as follow:
  • Fruits harvested on green variety and germinating with green sprouts are true-to-type green variety; those giving brown sprout are natural hybrids between green and red varieties. 
  • Fruit harvested on the red-orange variety and germinating with red-orange sprouts are true to type red-orange variety. 
  • All fruits with brown sprouting will be natural hybrids.

Figure 1. An example of use of genetic markers
to identify some coconut progenies by using the color of the sprout.

When planting together a yellow variety and a green variety, it is not possible to fully separate pure varieties from hybrids. For the yellow parent, the system works well: yellow sprouts are true-to-type to the yellow parent, green sprouts are hybrids. But on the green parents, almost all sprouts will be green whether the other parent is yellow or green: so the genetic marker does not allow to distinguish between true-to-type and hybrids for the green parent. A discussion on this topic was conducted on the coconut google group in December 2015. If a model based on two independent genes is considered, the cross of two green Talls may give some yellow palms. Anyway, the allele coding for the yellow colour is often very rare in natural Tall populations, except may be in some places of Papua New Guinea. 

Some farmers and scientist are able to separate Talls from Hybrids in the nursery, even when both are of green colours. This is for example done in Sri Lanka, where they use a Green Dwarf as female parent to produce hybrids with the Sri Lanka Tall (of mix Green and Brown coulours).

Another genetic markers is pink color that can be found inside the husk of unripe fruits, at the base of female flowers of just opened inflorescences, on the very young sprouts when just germinating, and in the tips of the roots. This marker is presently underutilized by scientist, but well known by some farmers: for instance, in French Polynesia, some farmers succeeded to select allogamous compact dwarfs with both big red fruits and this pink color marker; and also, in Raiatea Island, farmers bred some sweet-husked palms by using the same pink color marker.

Regarding aromatic coconut varieties, a special fragrance/aroma is present is the water of tender nuts, but it can also be smelt by crushing a small root, or by slightly heating a piece of leaflet with a lighter. This can also be done at the nursery stage. This is very useful to discard palms that are not aromatic in the progeny.

The economic value of such genetic markers is tremendously high. They serve in most systems of production of hybrids and dwarfs seednuts to separate the hybrids from the dwarfs. Both farmers and breeders are strongly encouraged to search for any additional genetic marker which could be used at the nursery stage. Such a discovery would benefit the entire coconut community.