Coconut - Genomics of Cultivated Palms - page 12

2

Coconut

2.1 Introduction

Coconut ( Cocos nucifera L.) is an important multipurpose perennial crop of the trop-
ics. Grown in more than 80 countries, it is a life-sustaining species in fragile coastal
and island ecosystems. Indonesia, Philippines, India, and Sri Lanka are the major
coconut-producing countries. Coconut belongs to the monotypic genus Cocos , under
the subtribe Butiinae; and tribe Cocoeae, subfamily Arecoideae. It is a diploid spe-
cies (2 n  2 x  32) with a large genome size of 2.15 Gb nucleotide pairs [1] . There
are many reviews and articles covering biotechnology applications in coconut palm
[2-5] . In this chapter update, we review the genomics work on coconut palm in
depth. Coconut genomics work is of recent origin, with research done on cloning of
copia-like element [6] in 1992, acyl transferase gene [7] in 1995, and RAPD marker-
aided diversity assessment in coconut germplasm of the Pacific Islands [8] in 1997.

2.2 Markers for Assessment of Diversity

Molecular markers (RAPD [8-11] ), AFLP [12,13] , RFLP [14] , ISTR [15] , ISSR
[16] , SSR [17-25] , WRKY genic SNP [26] and WRKY genic SSR [27] ), and bio-
chemical markers (polyphenols [28] , enzymes [29,30] , and proteins [31] ) were
employed to understand the diversity in coconut germplasm. Details of the materials
used, primers tried, and the conclusions are given in Tables 2.1a-2.1d .
Tall and dwarf are the two main habitual forms available in the coconut palm
[32] . Tall cultivars are predominantly heterogeneous due to their cross-pollinated
nature. Tall cultivars possess late-bearing (7-10 years) stems with long internodes,
produce large fruits, and grow very tall (20-30 m). Dwarf cultivars grow to a height
of 10-15 m and occur at very low frequency in nature. They are early bearing (5-6
years), predominantly self-pollinated, and thereby homogenous. They possess thin
stems with short, compact internodes and produce small fruits with thin copra [33] .
There are three types of dwarf forms in coconut. First is the javanica, intermedi-
ate types with semi-tall habit and medium-sized fruits; the second type, nana, are
real dwarf types with dwarf habit and small fruits. Hybrids involving tall and dwarf
forms gave early-bearing, high-yielding intermediate forms with medium-long inter-
nodes [34,35] . Lethal yellowing is a serious disease of coconut in Latin American
and Caribbean countries. Introduction of coconut germplasm in Ref. [36] led to
observation of the resistance to the disease in Malayan dwarfs. Hybrids between
dwarf and tall cultivars for high yield and resistance to lethal yellowing are the two
important achievements in coconut genetics.