| Summary: | Polyploidy has played an important evolutionary role in the genus <i>Festuca</i> (Poaceae), and several ploidy levels (ranging from 2n = 2<i>x</i> = 14 to 2n = 12<i>x</i> = 84) have been detected to date. This study aimed to estimate the genome size and ploidy level of two subspecies belonging to the <i>F. yvesii</i> polyploid complex by flow cytometry and chromosome counting. The phenotypic variation of the cytotypes was also explored, based on herbarium material. The genome size of <i>F. yvesii</i> subsp. <i>lagascae</i> has been estimated for the first time. Nuclear 2C DNA content of <i>F. yvesii</i> subsp. <i>summilusitana</i> ranged from 21.44 to 31.91 pg, while that of <i>F. yvesii</i> subsp. <i>lagascae</i> was from 13.60 to 22.31 pg. We report the highest ploidy level detected for <i>Festuca</i> (2n = 14<i>x</i> = 98) and previously unknown cytotypes. A positive correlation between holoploid genome size and chromosome number counts shown herein was confirmed. The morphometric approach showed a slight trend towards an increase in the size of some organs consistent with the variation in the ploidy level. Differences in characters were usually significant only among the most extreme cytotypes of each subspecies, but, even in this case, the high overlapping ranges prevent their distinction.
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