Trials were carried out from 1959 to 1967 on the secondary sexual characteristics of asparagus and on the inheritance of sex.
Growth rate of the seedling is closely dependent on the seed size. Seed size depends not only on number of seeds per berry but also it is genetically controlled independently of sex. Seeds giving rise to ♀plants germinate faster than those giving rise to ♂plants. Among 119 crosses 1000 grain weight varied from 15.4 to 28.7 g. In crosses of 17 ♀plants with 7 ♂plants it was evident that both parents, but particularly the ♀plants, control seed size. Crop observations on seedlings showed that the number of stalks developing was the same in ♂and ♀plants. ♂plants flower earlier.
In a field trial at Maasbree set out in the previous year no difference was observed initially in 1962 in the number of stalks in ♂and ♀plants. After berries had first been produced ♀plants lagged. ♂plants had more stalks, thinner stalks and a higher stalk product (sum of the squares of the diameters of all the stems of each plant) than ♀plants. From 1963 to 1965 production of ♂plants differed from ♀in:
a. a higher yield; the difference increased over the years from 27 to 48 %;
b. thinner spears; the average spear weight was about 10 % less;
c. more spears;
d. earlier production;
e. more spears at each harvest.
The observations on this field in the summers of 1963 and 1964 confirmed the results for 1962. ♀plants which produced a lot of berries in 1962 produced less stalks in the next three years than ♀plants producing few berries in 1962. Thus there is a tendency for berry production to inhibit stalk production.
Determination of sexes in plots varying from 1 to 100 years old showed that as the fields became older the percentage of ♂plants rose from 50 to 85. Thus over the years more ♀than ♂plants die. ♂plants have a longer life, as seen in their greater resistance to foot-rot. In severely diseased fields there are more ♂than ♀plants because more of the ♀plants have died.
Flowers of ♀plants hardly vary in the form and size of the rudimentary stamens, whereas ♂plants vary considerably in the development of the pistil. ♂flowers could be divided into five classes ranging from those with a tiny rudimentary ovary to a completely developed pistil. The latter could form berries. Flower structure is such that self-pollination occurs and hence self-fertilization. Plants with most flowers ♂can also have flowers of other classes; no plants were found with flowers only of the class with a fully developed pistil and stamens. On seedbeds and commercial fields ♂plants flowered more heavily than ♀; on very old fields ♀plants hardly ever flowered.
In a young field 50 % of the plants are ♂and 50 % ♀. However among crosses of 17 ♀plants with the same ♂plant, one ♀plant gave significantly more ♂than ♀offspring.
As ♂plants were more productive than ♀plants, ways were sought of sexing the seedlings before planting out to allow entirely ♂beds to be set out. Two year-old ♂plants can be planted out but the work is laborious and the plants take poorly. The difficulties cannot be solved by selecting seedlings by weight and by recovery after removing the tops or by sexing the seedlings with potassium chlorate. When the material is pollinated with old pollen, the percentage ♂plants exceeds 50 %. The size of X and Y pollen grains was the same.
As the trials failed to give a practical method of obtaining an entirely ♂field, it was decided to resort to genetics. Both ♂and ♀plants occur in the fields. The obligate cross-fertilization yields a population of XX and XY in the proportion 1 : 1. There may be an occasional andromonoecious XY plant. With self-fertilization the offspring is XX, XY and YY in the proportions 1 : 2 : 1. These YY plants are necessary to produce entirely ♂offspring (XY) by crossing with ♀(XX).
XY and YY plants cannot be distinguished phenotypically; this is only possible by test crosses with ♀plants.
Seeds giving rise to YY plants germinate more slowly than those developing XY plants. The YY plants tend to start flowering earlier, although this difference is not significant. The XY and YY plants are the same in average pollen grain size.
Cytological studies showed that asparagus has 10 pairs of chromosomes. In somatic root-tip cells the 10 pairs can be divided into 3 groups: long, medium and short. Five pairs are long, one is medium and the other four are short. One of the five pairs of long chromosomes has a satellite. One author called this pair the sex pair. Despite careful observations on this pair of chromosomes in XX, XY and YY plants, I found no differences. It was impossible to distinguish the sex chromosome in the somatic cells of root tips.
As stated before the ♂flowers are divided into 5 classes. If many plants have to be studied, this method is quite time-consuming. Hence the classification is based on berry production, for this procedure requires less time. The ♂and andromonoecious plants were divided into 4 groups:
a. purely ♂, no berries;
b. slightly andromonoecious, 1-10 berries per plant;
c. moderately andromonoecious, 11-100 berries per plant;
d. strongly andromonoecious, over 100 berries per plant.
Development of the pistils in andromonoecious plants and consequently berry production depend upon day-length and temperature. Short days and low temperatures promote pistil development. The position of the flower on the plant does not effect development of the pistil. The degree of andromonoecy depends on the plant's age as well as on external influences. In general the plant is most andromonoecious at maximum growth, which is achieved when three years old. They become less andromonoecious as the plants grow older.
Strongly andromonoecious plants resemble ♀in crop development, both groups being characterized by few stalks per plant.
There is no difference in average pollen grain size between andromonoecious XY, ♂XY, andromonoecious YY and ♂YY plants. Gibberellic acid was unsuccesful as a spray to stimulate te development of the ovary in ♂flowers and the stamens in ♀flowers.
The tests of inheritance of andromonoecy can be divided into 3 groups:
a. crosses of some YY plants with one ♀plant and of various ♀plants with one YY plant;
b. segregation in I 1
, I 2
and I 3
and in the progeny of crosses of I 1
and I 2
plants with one ♀plant;
c. segregation in I 3
and I 4
and in the progeny of reciprocal crosses of I 2
and I 3
The results of tests a and b gave rise to the following hypothesis. The autosomes of ♀and ♂plants possess genes with a weak to strong female influence. These genes, called G, have no influence in ♀plants, because they already possess berries, or in ♂plants, because these have the genes AA and consequently produce no berries. But if the plant has the genotype XYAa a few berries are produced, as the G genes may manifest themselves. Inbreeding of it yields the following offspring:
XYAa slightly andromonoecious, if G factors are present
XYaa strongly andromonoecious, if G factors are present
YYAa mostly ♂
YYaa andromonoecious, if G factors are present
The following genotypes could be called extreme:
strongly ♀ not visible in the phenotype
slightly ♀ not visible in the phenotype
After self-fertilization the XYAa plants can segregate into ♀, andromonoecious and ♂plants in the proportion 4 : 7 : 5. If growing conditions are optimum the YYAa plants with their two Y chromosomes can segregate into andromonoccious and 3 plants 1 : 3; sometimes, however, YYAa can be slightly andromonoecious. The degree of andromonoecy can be influenced in various ways, e.g. by the difference in number of G factors. Crossed with one ♀plant the YY plants (progeny of selffertilized I 1
XYAa plants) can segregate into 1 AA : 2 Aa : 1 aa; the first group gives purely ♂progeny, the second slightly to moderately andromonoecious and the third group strongly andromonoecious.
The segregation of the crosses of some ♀plants with one YY plant can be explained by assuming one or more pairs of genes influencing the degree of andromonoecy. The influence of these genes depends largely upon environment.
Test (segregation in the I 3
and I 4
and in the progeny of reciprocal crosses of I 2
and I 3
plants) showed that germination of pollen decreased in 4 of the 5 inbred lines. The average pollen germination of the inbred progeny of one XY plant and one YY plant is 3,7 and 1,6 %, respectively. These inbred lines include various plants whose pollen did not germinate at all so that inbreeding depression results in male-sterile XY and YY plants.
Although the pistils were well formed, few berries were produced because the pollen of inbred lines germinated poorly and of crosses only moderately. (These tests were all in a greenhouse and hence the flowers were self-pollinated).
Although as a result of repeated inbreeding the plants used in this test would possess the genotype aa and many G factors, the degree of andromonoecy in the inbred lines and crosses did not come up to expectation, probably because the observations were on young plants in the greenhouse. There are differences in number and degree of andromonoecy between reciprocal crosses. As both the genetic constitution of the reciprocal crosses and the growing conditions of the plants are the same, andromonoecy tends to be influenced cytoplasmatically.
Despite a thorough search among ♀plants no gynomonoecious counterparts of the andromonoecious plants could be found.
Instructions were drawn up for the production of ♂material from the hypothesis of how andromonoecy is inherited. As starting material XXAA and YYAA plants are needed, whatever the number of G factors. A male variety may be produced as follows. First search is made for XXAAGG ..
plants, which are needed to trace YYAA plants, irrespective of G factors, in the progeny of self-fertilized XYAa or YYAa plants. The YYAA plants are then crossed with many productive ♀plants. Productive F 1
plants are subsequently traced by observing flowering and yield of spears. Largescale production of this F 1
requires vegetative propagation of the XXAA and YYAA, e.g. by rootstock division. In the course of several years 1596 and 500 divisions respectively, have been obtained from 14 ♀and 3 ♂plants.
An entirely ♂3 F 1
, can be produced from inbred lines. The best plan for inbreeding is to self-fertilize XYAa plants which give rise to XXAA and YYAA offspring. When self-fertilized the XYAa segretates into: 1/16 XXAA, 2/16 XXAa, 1/16 XXaa, 2/16 XYAA, 4/16 XYAa, 2/16 XYaa, 1/16 YYAA, 2/16 YYAa and 1/16 YYaa. After inbreeding for five or six generations entirely homozygous XXAA and YYAA plants can be produced.