Sorbocotoneaster – a source of apomixis in Sorbus breeding

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Abstract
Creation of self-fertile and highly winter-hardy varieties of fruit crops for regions with risky farming, such as Siberia, is a priority task. The native species Sorbus sibirica is characterized by cross-pollination and periodic fruiting, which makes the search for sources of apomixis relevant. The aim of the work was to study the ability of self-fertility in representatives of the hybridogenic genus Sorbocotoneaster and assess its potential for rowan breeding. We studied natural and artificial allopolyploid forms of Sorbocotoneaster with different genome compositions (SSCC, SSSC, SSC, SCC), as well as the parental species S.sibirica (2n = 2x, SS) and C.melanocarpus (2n = 4x, CCCC). The fruit set was studied under different pollination conditions: autonomous apomixis (without pollination), self-pollination, open pollination, cross-pollination with mountain ash pollen and foreign pollen. The clonality of the seed progeny was confirmed by electrophoresis of cotyledon storage proteins. It was shown that the allotetraploid form of Sorbocotoneaster (SRCT-JAK-1501, 2n = 4x, SSCC genome) has obligate autonomous apomixis, demonstrating fruit set at the level of 27.7% without pollination. With self-pollination and pseudogamous pollination, the fruit set reached 54 – 55%. Complete homogeneity of the electrophoretic spectra of proteins and the morphology of the seed progeny confirmed their clonal origin. Tetraploid hybrids with the SSSC genome also showed high fertility, while triploid genotypes (2n = 3x) were low-fertile. Thus, Sorbocotoneaster pozdnjakovii is a unique source of obligate apomixis, high winter hardiness and self-fertility. Incorporating Sorbocotoneaster into rowan breeding programs will allow creating genotypes with a guaranteed stable yield regardless of weather conditions during the flowering period, which is critically important for horticulture in the extreme conditions of Siberia.

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