| Summary: | Background and Objectives:<b> </b>Polyploidisation and frequent hybridisation play an important role in speciation processes and evolutionary history and have a large impact on reproductive systems in the genus <i>Crataegus</i>. Reproductive modes in selected diploid and polyploid taxa in eastern Slovakia were investigated and analysed for the first time. Materials and Methods: Diploid, triploid, and tetraploid hawthorns were tested for self-pollination, self-compatibility, and self-fertilisation. Pollination experiments were performed within and between diploid and triploid species to determine the possibilities and directions of pollen transfer under natural conditions. Seeds from crossing experiments and open pollinations were analysed using the flow cytometric seed screen method. Results: These experiments demonstrated that sexual reproduction, cross-pollination, and self-incompatibility are typical of the diploid species Crataegus<i> monogyna</i> and <i>C. kyrtostyla</i>. Seeds produced by self-fertile tetraploid <i>C. subsphaerica</i> were derived from both meiotically reduced and unreduced megagametophytes. Conclusions: Experimental results concerning triploid <i>C. subsphaerica</i> and <i>C. laevigata</i> × <i>C. subsphaerica</i> are ambiguous but suggest that seeds are almost exclusively created through apomixis, although a few sexually generated seeds were observed. In the genus <i>Crataegus</i>, pseudogamy is a common feature of polyploid taxa, as in all cases pollination is essential for regular seed development. Research Highlights: We suggest that all studied <i>Crataegus</i> taxa produce reduced pollen irrespective of ploidy level. Moreover, we emphasise that triploids produce apparently aneuploid pollen grains as a result of irregular meiosis. They are also capable of utilising pollen from 2<i>x</i>, 3<i>x,</i> or 4<i>x</i> donors for pseudogamous formation of endosperm.
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