Malus sieversii is a wild apple native to the mountains of Central Asia in southern Kazakhstan. It has recently been shown to be the primary ancestor of most cultivars of the domesticated apple (Malus domestica). It was first described as Pyrus sieversii due to its similarities with pears in 1833 by Carl Friedrich von Ledebour, a German naturalist who saw them growing in the Altai Mountains.
Malus sieversii grows in many different habitats. They prefer high temperatures and short winters, but they are also found in the Tian Shan Mountains with long and harsh winters. They are distributed mainly within the Yili valley as the damp climate is suitable for their growth.
It is a deciduous tree growing , very similar in appearance to the domestic apple. Its pollen grains vary in size and are seen to be ovular when dry and spherical when swelled with water. Its fruit is the largest of any species of Malus except domestica, up to 7 cm in diameter, equal in size to many modern apple cultivars. Unlike domesticated varieties, its leaves go red in autumn: 62.2% of the trees in the wild do this compared to only 2.8% of the regular apple plant or the 2,170 English cultivated varieties. The species is now considered vulnerable to extinction.
Characteristics and growth
M. sieversii has the capability to reproduce vegetatively as they form root suckers, or basal shoots. The clonal individual grows from the adventitious bud on the root, with identical genetic materials to the mother plant. It was originally believed that wild apples produce root suckers only when upper plant parts are damaged. However, more evidence suggested root-sucker growth occurs in healthy plants as a dispersal aid.
For wild apples, proper development of root suckers requires certain humidity and aeration levels in the surface soil, where the mother root is located. Successful root-sucker growth also depends on shoot arrangement, time of growth and health conditions of the mother plant.
Genetics of self-incompatibility, the system for preventing self-fertilization in angiosperms, have also been studied for M. sieversii. Its genetic diversity in relation to self-incompatibility is substantially less when compared to its close relative, Malus sylvestris. Although M. sieversii lacks this diversity, it can survive in the wild without intervention as long as no more diversity loss occurs. The leading theory for this lack of diversity is due to a major population bottleneck during the Last Glacial Maximum which caused wild M. sieversii populations to scale back into a smaller area within the Yili Valley.
Growth cycle
The growth cycle of M. sieversii could be divided into several stages from germination to developing fruit bearing trees, and to the death of aged trees.
Period I starts from seed germination to the development of vegetative tree parts, and to the first round of fruiting. A typical apple tree reaches age 6 to 8 in this period. Prominent primary growth and a significant number of root sucker growth are seen in this period. Period II involves more growth and fruit bearing. Wild apple trees usually reach age 10 to 12 in this period. The number of fruits increase significantly as prominent secondary growth and branching take place. Period III involves more growth and fruit bearing. Wild apple trees enter regular fruiting and reach maximum fruit bearing in this period, and usually reach age 25 to 30. Decreased branching rate, and less growth of crown periphery are seen. A lot of vegetative growth is contributed by basal shoots (root suckers). Period IV is fully fruit bearing. Development of trees fully ceases in this stage and they reach around 60 to 70 years of age. Branching decreases as they slowly die off from the base to the periphery. After age 60 to 70, trees experience rapid desiccation of the branches, then death.
History and importance
Malus sieversii has previously been identified as the main contributor to the genome of the cultivated apple (Malus domestica), on the basis of morphological, molecular, and historical evidence. Fruit traits including crispness, more flavour intensity and fruit weight have undergone differential selection by humans to produce Malus domestica as seen today. The dispersal of M. sieversii and its progeny throughout history can be attributed to the Silk Road. A DNA analysis in 2010 confirmed M. sieversii as the progenitor of the cultivated apple. It has a highly variable genetic diversity therefore it is the genetic source for abiotic and biotic stress tolerance, many disease resistance and unique fruit traits.