Title : Attempts to explain incompatibilities in the process of pollination and fertilization of blackberry flowers (Rubus L. subgen. Rubus)
Abstract:
Understanding the biology of flowering among studied blackberry genotypes derived from Niwa’s berry breeding program.
The course of pollination and fertilization of flowers of 5 cultivars and 15 breeding clones of blackberries were examined in this study. Blackberries are commercialized species within Rubus (Rosaceae). They are classified in the Rubus subgenus Rubus (formerly Eubatus) and typically don’t have a species epitaph because the cultivated blackberries are nearly all derived from at least two or more species. Chromosome numbers in Rubus range have substantial variation in the wild and cultivated genotypes.
Reports provided information for this range from 2n=2x=14 to 2n=18 x =126 including odd-pliods and aneuploids (Thomson 1995 a, 1995 b, 1997, Meng and Finn 1999). Cultivated blackberries all contain multiple species in their backgrounds (Clark et al.2007) and thus variation in progenitor species chromosome number. The Rubus subgenus Rubus contains only 10% of diploid species. Most blackberries are tetraploids 2n = 4x = 28. Thus, blackberries are polyploids that tend to produce apomictic seeds (Gustafsson 1943 and Nybon 1988). The most common species and varieties are self-fertile, and some also self-pollinating, through the special construction of anthers (Nybom 1986 a, b). Embryos can be formed both from the egg cell through fertilization, as well as from diploid cells of ovule which give rise to the adventitious embryo.
In this case, there may be two embryos (Pratt and Einset 1955, Czapik 1983). Apomictic forms of blackberries are self-fertile and autogamous i.e pollination of foreign pollen induces the formation of apomictic embryos, (the need for the formation of endosperms) Nybon 1987. The fact that the resulting seeds are not dependent on the donor pollen report Haskell 1960, Jennings and Topham 1971.
The diploid varieties show no deviations in the meiosis process. Such abnormalities were observed in polyploid species (Pratt, Einset 1955). The flowers have a diameter of 2 to 4 cm, the petals of the crown are white or pink. They have a large number of stamens (from 50 to 200) and about 50 to 100 pistils (Strik and Finn 1996). Two ovules grow in the ovary, one disappears before flowering, just as with all drupes, one remains as the embryo sac Polygonum type. After fertilization, the ovary transforms into single fruit, several dozen of which form an aggregate fruit structure. They may even form a single ovule on the receptacle. The amount of seeds is strongly correlated with the viability of pollen. There is a high correlation between the viability of pollen and the amount of seeds (Nybon 1985). On the pollinated stigma there are from 50 to 200 pollen grains that germinate after 30 minutes, but most often after a few hours. After 18 hours, the pollen tube is growing to the base of the style. Fertilization is recorded after about 30 hours (Engelhardt and Stösser 1979).
In many older breeding works, great attention is paid to the ratio of the amount of good pollen to the number of ovules. Blackberries belong to the transitional species between kleistogamy and allogamy. They develop a rather less valuable pollen, which obviously affects the amount of seeds (Cruden 1977).
Attempts to explain incompatibilities in the process of pollination and fertilization of blackberry flowers (Rubus L. subgen. Rubus)
There are three dominant types of problems or incompatibilities that there are revealed in that research. These are sporophytic incompatibility, anatomical incompatibility caused by narrowing of the transmission tissue of the style, and incompatibility of selective fertilization, resulting from irregularities in the course of micro and macrosporogenesis. Among all of the tested varieties, only two of those got proper fertilization. Among all varieties being tested, the fertilization of the ovules ranged from 0 to 80%. ‘Prime-Ark ®Freedom’ and ‘Apache’ varieties are self-fertile, while ‘Prime-Ark® 45’ and ‘Reuben’ are not self- pollinating.
Impact of revealed data during the study on blackberry breeding program carried out in the Niwa Berry Breeding Ltd.
The purpose of the research is to show that fertilization of the ovules is a decisive factor in the selection of appropriate clones for further cultivation of blackberries. The presented method is easy and fast, and the assessment is very reliable. At the same time, we try to present the course of pollination and fertilization of blackberry flowers, taking into account the incompatibilities that occur in this process. The presented method of fertilization of ovules can significantly accelerate breeding work.
