Evolutionary History of the Genus Pistacia (Anacardiaceae)
Pistacia L. belongs to the family Anacardiaceae (cashew family), order Sapindales. Pistacia vera L. (cultivated pistachio) is by far the most economically important species in the genus. It has edible seeds and considerable commercial importance. The evolutionary history and the phylogenetic relationships between species within the genus are not well understood. A better understanding of these relationships is needed to make the species more useful for plant improvement or genetic studies. The objective of this perpestective is to provide additional insight into understanding the evolutionary history of Pistacia. In conclusion, Pistacia is a monophyletic genus and it contains two sections (Lentiscella and Pistacia) and it is originated in the Paleocene epoch. This is based on Anacardiaceae being pantropical in distribution with North and South America representing major diversification centers of the family including the geographical distribution of Pistacia. This perspective provides additional insight into understanding the evolutionary history of the genus Pistacia to make the species more useful for plant improvement or genetic studies.
Pistacia L. belongs to the family Anacardiaceae (cashew family), order
Sapindales (Stevens, 2008). It contains nine species
and five subspecies according to the current study completed by AL-Saghir
and Porter (2006) (not published). Species are xerophytic trees, deciduous
or evergreen and dioecious, up to 8-10 m high. Leaves are pinnately-compound,
with broad, elliptical to round-ovate leaflets. Buds are single, apical and
usually vegetative. In both male and female trees, flowers are apetalous, subtended
by 1-3 small bracts and 2-7 bracteoles and borne in racemes or panicles. Male
flowers have 4-5 anthers inserted on a disc. Female flowers have a short, 3-fid
style. The species are wind-pollinated and the fruit is a drupe (Zohary,
Pistacia vera L. (cultivated pistachio) is by far the most economically important species in the genus. It has edible seeds and considerable commercial importance. The other species grow in the wild and their seeds are used as a rootstock seed source and sometimes are used for fruit consumption, oil extraction, or soap production.
The pistachio is native to the arid zones of Central Asia; it has been cultivated
for 3000-4000 years in Iran and was introduced into Mediterranean Europe by
Romans at the beginning of the Christian era (Crane, 1978).
Pistachio cultivation extended westward from its center of origin to Italy,
Spain and other Mediterranean regions of Southern Europe, North Africa and the
Middle East, as well as to China and more recently to the United States and
Australia (Maggs, 1973; Hormaza et
al., 1994, 1998). Pistacia vera is the
only species in this genus that is successfully grown in orchards; it produces
edible seeds large enough to be commercially acceptable. Pistachios are adapted
to a variety of soils and are probably more tolerant of alkaline and saline
soil than most tree crops (Tous and Ferguson, 1996).
Moreover, Pistachios thrive in hot, dry, desert-like conditions. Pistachios
are utilized for the most part in the shell for fresh consumption; processed
uses include candy, baked goods and ice cream. They also have folklore, medicinal
and non food uses such as toothache relief. The resin is used for gum (dried
resin) and as a blood-clotting agent in Europe and the Middle East. In India,
the husks are used for dying cloth and tanning hides. Pistachios have been reported
as a remedy for scirrhus and sclerosis of the liver, abscesses, poor circulation
and other medical problems (Tous and Ferguson, 1996).
Currently, Iran, the United States, Turkey and Syria are the main Pistachio
producers in the world, contributing over 90% of the world production (FAO,
The current study supported the monophyly of Pistacia. The genus divided into two monophyletic groups: One group (Section Pistacia) contains P. atlantica Desf., P. chinensis Bunge, P. eurycarpa Yalt., P. falcate (Bess. ex Martinelli) Rech. f., P. integerrima (J.L. Stew. ex Brandis) Rech. f., P. khinjuk Stocks, P. mutica Fisch. and C.A. Mey., P. palaestina Boiss., P. terebinthus L. and P. vera L. while, the other group (Section Lentiscella) contains P. aethiopica Kokwaro, P. lentiscus L., P. mexicana Humb., Bonpl. and Kunth, P. texana Swingle and P. weinmannifolia Poiss. ex Franch.
Zohary (1952) noted that evidence based on fossil,
P. lentiscus originated 40 million years ago and the genus as a whole
probably developed more than 80 million years ago. This conclusion is questionable
since Anacardiaceae pollen and wood first appear in the Paleocene epoch, 65
to 55 million years ago (Hsu, 1983; Muller,
1984) and is found throughout the world. The origin for the order in which
the Anacardiaceae occurs, Sapindales, dates back approximately 84 to 65 million
years before present (Magallon and Sanderson, 2001;
Wikstrom et al., 2001).
Derived from this information and the fossil records of the genus, I hypothesize
that Pistacia originated in the Paleocene epoch. Since, Anacardiaceae
is pantropical in distribution and North and South America represent major diversification
centers of the family and given the geographical distribution of Pistacia,
I postulate that ancestral species of Pistacia came from North America.
This hypothesis is supported by Pistacia fossil records from the Paleocene
of Wyoming and Colorado (Edwards and Wonnacott, 1935).
Migration may have taken place from western Laurasia (North America) to Eastern
Laurasia (Europe and Asia) ending up in Central Asia via Europe where the genus
radiated within Asia (West Asia and Mediterranean Basin) as hypothesized by
Weeks et al. (2005) for the Burseraceae. This
migration may have been facilitated by the boreotropical land bridge (Tiffney,
1985; Tiffney and Manchester, 2001), which spanned
the North Atlantic during the early to middle eocene. Global temperatures during
the Eocene were highest during this time period and tropical vegetation is known
to have occurred in this land corridor (Wolfe, 1978;
Zachos et al., 2001). Cooler temperatures during
the Middle Eocene extirpated frost tolerant taxa in this region and the physical
land connections disappeared sometime afterward (Weeks
et al., 2005).
This vicariant scenario for Pistacia is indirectly supported by the
localities of Pistacia fossils from the Early Eocene in England and Russia
(Weeks et al., 2005). Following the migration
of ancestral Pistacia into the Old World, Pistacia appears to
have dispersed and radiated within continental Africa during the Middle Eocene
(44 Ma). The spread of Pistacia to India and Southeast Asia appears to
have occurred in relatively recent geologic time (5.0 Ma), perhaps due to a
Northeasterly range expansion of Pistacia coincident with the establishment
of arid habitat in East Africa (Potts and Behrensmeyer, 1992;
De Menocal, 1995).
The Oligocene origin of the American species, Pistacia mexicana, from
a Southeastern Asian ancestor (like P. weinmannifolia ) may be due to
long distance dispersal or migration through probable trans-Atlantic long distance
dispersal (Renner, 2004).
I posit that the genus extended its distribution range away from Central Asia
to West Asia and the Mediterranean basin, East Africa and the new world species
by passive dispersal mediated by wind, water, birds or even by people. This
is supported by evolution toward a smaller seed with a hard endocarp paralleling
a change in reproductive strategy from distribution by ground squirrels (burying
the seed, as with walnuts and oaks) to bird- or wind-mediated distribution,
which would require a seed capable of passing through a birds digestive
system or being blown by the wind. (Jordano, 1989).
Species in both Section Lentiscella and Pistacia, which diverged
relatively early, have evolved smaller leaves with more leaflets and a winged
rachis and smaller hard seeds although these monophyletic groups probably evolved
independently and gradually. Smaller elongated leaflets with pointed shoot apices
also are more efficient for water removal from the leaf surface compared with
simple rounded leaves and are more adequate for wind pollination. This would
be a useful adaptation as the genus moved into higher rainfall regions (Parfitt
and Badenes, 1997).
Li and Tanimura (1987) suggested that differences in
mutation rates among organisms may be more a function of generation time than
DNA repair rates. Pistacia species have a long generation cycle of at
least 10 years to first flowering and a life-span estimated to be as much as
400 years in some cases. The average replacement cycle for pistachio is probably
between 50 and 200 years in the wild, so it is not surprising that Pistacia
has evolved much more slowly than the annual species used to derive standard
mutation rate estimates (Parfitt and Badenes, 1997).
Zohary (1952) hypothesized that P. khinjuk to
be directly descended from P. vera, a hypothesis that can not be supported
or rejected so far. Zohary (1952) also considered P.
khinjuk and P. vera to be the most primitive, each showing nine characters
attributed to primitive species: simple leaves, odd-pinnate leaves, small number
of leaflets per leaf, symmetrical leaflets, rounded leaflet apex, simple petiole
(no wings), highly branched panicles, deciduous character and large fruit. This
is consistent with a Central Asian center of diversity for the genus given that
the natural range for P. vera spans this region. Pistacia
species based on P. khinjuk and P. vera are the only Pistacia
species with large edible drupes. Both species have a similar somewhat unique
three-leaflet odd-pinnate leaf. Random Amplified Polymorphic DNA (RAPD) study
completed by AL-Saghir and Porter (2006) indicated that
P. khinjuk and P.vera form a very close pair, accordingly,
I posit that P. khinjuk is indeed a direct descendant of P. vera.
This perspective provides additional insight into understanding the evolutionary history of the genus Pistacia to make the species more useful for plant improvement or genetic studies.
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Published on the internet. (Accessed 1 October 2005). http://www.fao.org