Asparagus

Asparagus has been a valued vegetable since its early domestication, not only for
human consumption, but also for its medicinal properties. Nowadays its cultivation is
spread in all continents with steady increments of the planted area due to an
increased consumer demand for fresh, canned and frozen asparagus (see Table 1).
The largest increases in asparagus production in the last ten years have occurred in
countries like Peru and China, which have low labour rates and/or the possibility of
marketing their production when prices are high in other countries or hemisphere
(Benson, 2002a)

Some seed companies have taken advantage of this expansion, with the need and
development of hybrids adapted to newly production areas and cultivation strategies;
however, the development of new varieties highly productive, tolerant to the most
frequent diseases and high market quality will be the challenge of breeders in the
coming years.
Many symposia, thesis and research papers have came out since Ellison (1986),
last chapter devoted to asparagus breeding in a vegetable breeding book, 20 years
ago. Our wish is to reinforce some of the previously stated concepts and review the
best strategies to secure highly valuable outputs from a breeding program.

Cultivated Species and Domestication
Asparagus is a large genus comprising about 150 species of herbaceous perennials,
tender woody shrubs and vines. Some of them are grown for their ornamental value
and foliage, and one of them (A. officinalis L.) for food (Bailey, 1942). Three subgenera, Asparagus, Protoasparagus and Myrsiphyllum can be recognized according
to Clifford and Conran (1987). The species of the first subgenus are dioecious, while
those of the other subgenera are hermaphrodite. Asparagus species are naturally
distributed along Asia, Africa and Europe. Many of them have economic value as
ornamentals (i.e. Asparagus plumosus, A. densiflorus, A. virgatus), or for their medicinal properties (i.e. A. racemosus, A. verticillatus, A. adscendens) (Štajner et al., 2002).
Even when the search for young tender shoots as a tasty vegetable of the wild species
A. acutifolius has cultural roots in Spain and Greece (Ellison, 1986; GonzálezCastañón, per. comm.), the only worldwide cultivated species for tender shoots either
blanched (white) or light exposed (green) is A. officinalis (hereafter referred as
asparagus or cultivated asparagus).
The postulated region of origin of asparagus comprises Eastern Europe,
Caucasus, and Siberia (Sturtevant, 1890), where supposedly was domesticated.
Greeks and then Romans took the culture of growing asparagus from eastern nations,
from which they also took the old-Iranian word ‘sparega’, which means shoot, rod,
spray; becoming ‘asparagos’ and ‘asparagus’ in Greek and Latin respectively. One of
the first detailed guides on how to raise asparagus is traced back to about 65 A.D. by
the Roman Columella (Lužný, 1979). Romans spread the culture of growing
asparagus along with their empire throughout Europe. There is also evidence that
crusading troops brought asparagus seeds from Arabian countries to the Rhine valley
around 1212 (Reuther, 1984). In all Europe, except Spain (Knaflewski, 1996), the
decline of the Roman Empire brought a decline in its cultivation, which was
confined only to some feudal lords and monastery gardens as a medicinal plant, until
the Renaissance, when it was rediscovered as an appreciated vegetable (Lužný,
1979).

General Botany
The perennial part of the plant is the rhizome (crown), which is composed of clusters
of buds with primary fleshy (storage) and secondary fibrous (absorbent) attached
roots. The buds sprout rendering the edible organ, a tender growing shoot (spear)
between 18-25 cm long, either blanched or not. Once harvests are discontinued,
shoots continue to grow becoming the aerial part of the plant (fern), which is
responsible for the replenishment of carbohydrates and accumulation until the next
harvest season is conducted. The full expanded stems, between a few to 50 or more
per plant depending on age, sex and cultivar, have long internodes and can vary in
height between 30 and 200 cm. Each stem contains primary and secondary branches
where flexuous cladodes (10-25 mm) are disposed in whorls.
In normal temperate field conditions flowering starts at the second year from seed
germination, but some plants can flower at the end of the first year. Flowering is in
flushes, with up to three flushes per year. Anthesis in each stem begins once it is
almost expanded, following an apical direction in the main stem and in primary and
secondary branches. Depending on air temperatures, blossom in each stem can last
up to two weeks.
Normal sex ratio in out-bred populations and cultivars is 1:1 staminate (male) to
pistillate (female) plants; however some hybrid cultivars are composed of entirely
male plants. Yellow-reddish male flowers (5-6 mm) and yellow-greenish female
flowers (c. 4 mm) are disposed in bundles of two or three flower per node, rarely
mixed with cladodes (Valdes, 1980). Natural pollination is conducted by bees and
bumblebees, and normally male plants flower earlier and produce many more
flowers than females. Fruits, reddish berries when mature, bear up to ten (normally
6-8) round black seeds. A 3-year-old or older female plant produces more than 2000
flowers and has the potential to produce more than 10.000 seeds (Machon et al.,
1995).

Dioecy in Asparagus
It is important for the plant breeders to consider the dioecism from an evolutionary
point of view. It will help to understand the phenomenon of the different genders and
proportions observed in populations and progenies, and to what extent genders are
associated to morphological attributes.
It is considered that asparagus has evolved from a primitive hermaphrodite form,
via an intermediate gynodioecy state, to the actual dioecious or subdioecious
populations, depending if andromonoecious plants (those bearing male and bisexual
flowers), are observed (Galli, et al., 1993). The theory tells us that the suggested
pathway (Charlesworth and Charlesworth, 1978) involves first a mutation for male
sterility, in the case of asparagus this gene being recessive (X). This mutation will
spread either if the female and hermaphrodite have the same fertility, but some rate
of selfing and inbreeding depression occur in the cosexual form, or if no selfing and
inbreeding depression are present but some gain in fertility (ovule production) is
gained in the female by allocation of resources in comparison to the hermaphrodite
state (Carlesworth and Charlesworth, 1978; Charlesworth, 1999). In asparagus, bisexual flowers are mostly self-pollinated (Thévenin, 1967, Galli et al., 1993); and some inbreeding depression was shown after continued selfing of andromonoecious
plants in comparison to unrelated outbred-cultivars (Ito and Currence, 1965) and in
the comparison of a Hybrid F1 cultivar UC 157 (cross of two heterozygous selected
plants) and the so-called F2 progeny (first generation of full-sibs, F = 0.25) (Farías
et al., 2004). Regarding to the fertility of female vs. hermaphrodite plants, no strict
evidence is available for asparagus, however among females some studies showed
association between fitness and morphological attributes; that is, significant positive
correlations were found between spear size and seed production (Currance and
Richardson, 1937); between stalk height and diameter, and total fruit weight (López
Anido, 1996); and between stem height and diameter, positively, and stem number,
negatively, with number of berries and seeds per berry (Machon et al.,1995).
Once the gynodioecious state (females and hermaphrodites) has been reached, a
second mutation for female-sterility is necessary to reach full dioecy. This mutation,
often called modifier, will spread as long as it confers increased pollen production in
comparison to the cosexual form. Again there is no clear evidence in asparagus
referring the gained pollen output of males vs. cosexuals, however it has been
noticed that male flowers are longer than cosexual flowers (Lazarte and Palser,
1979), and this feature has been associated in general to an increased male fitness
(Lloyd and Webb, 1977). The modifier can arise tightly linked to the previous male
sterility loci or to some extent of independence when it behaves hypostatically, that
is not affecting females.
Franken, (1970) studied several selfed progenies of andromonoecious plants and
concluded that a partial dominant gene (modifier, A in his nomenclature) was
responsible for the suppression of pistil development (see Table 2); and this gene
was inherited independently of the X male sterility locus, previously defined by Rick
and Hanna (1943) as a simple Mendelian factor mode of gender inheritance.

Franken (1970) proposed model is also in concordance with the results of Galli et
al. (1993) who, after analyzing the length of pistils in some backcrosses, concluded
that the factors affecting style length and stigma development (modifiers) are not
localized on the chromosome possessing the X locus; moreover, the backcross
distribution of style length fitted a model of at least two loci.
After a computer simulation, the model of an independent modifier gene for
female sterility (A) (with no effect on females) arose in a gynodioecious population
would spread to fixation, giving a population consisting of females together with
either males or modified hermaphrodites depending on the phenotypic effect of the
gene (Charlesworth, 1999).

In asparagus, when found, the percentage of andromonoecious plants is
extremely low, ranging from 0.1 % (Thévenin, 1967) to 1 % (Sneep, 1953). This is
suggesting a high frequency of the modifier gene (A) as observed by Franken (1970),
and turning the species as if it were segregating for only one gene affecting sex as it
was proposed in the earliest model of inheritance (Rick and Hanna, 1943).
The search of the so called supermales bearing YY, in the progeny of selfed
andromonoecious (XY) plants was postulated by Rick and Hanna (1943) in a way to
obtain a progeny constituted of entirely males plants. However due to the more
complex inheritance of gender in asparagus, as we have seen in the previous
paragraphs, extensive progeny tests are required to avoid the presence of
andromonoecious plants in the progeny of supermales; that is the super males have to
be YYAA in order to obtain a strict 100% male progeny (Sneep, 1953).