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Strength and (mal)adaptive consequences of cross-ploidy introgression

 

Polyploidy (the presence of multiple genome copies in a cell) is widespread across the plant kingdom, despite it representing a severe mutation with numerous genetic, physiological and ecological consequences for plant life. What are the key mechanisms allowing a newly formed polyploid to succeed? Gene flow from its diploid relatives could help the polyploid to gain additional variation and to establish itself in nature. In contrast to recent discoveries of ubiquity of gene flow in adaptation, we know little about this process in polyploids, mainly due to the technical challenges of population genomics in polyploids. By studying Arabidopsis arenosa, naturally encompassing both diploid and its autotetraploid derivative, we may overcome many of these obstacles. Using genomic resequencing, manipulated crossings and embryology we aim to characterize the permeability of the ploidy barrier. We will do this over multiple natural populations as well as different regions of the genome, and will test for any potential adaptive value of cross-ploidy introgression.

 

Using field sampling and flow cytometry we screened multiple mixed-ploidy (2x+4x) populations across three natural ploidy contact zones, but found no triploid (3x) adult hybrids and two 3x seedlings germinated from in situ collected seeds. This is not due ecological separation as we found no distinction in local habitat preferences among the ploidies. To test whether such lack of hybrids is due to intrinsic barriers that take part after pollination we performed experimental crossings. Initial results confirmed that the majority (~90 %) of the inter-ploidy crossing progeny was triploid, although we also found a non-negligible proportion of tetraploid hybrids (~7 %), documenting the possible role of unreduced gametes in mediating gene flow. We confirmed reduced fertility of the interploidy crosses compared to the controls with respect to several measurements involving seed production and germination success. We used confocal microscopy and explored endosperm development in seeds produced from interploidy crosses (Fig.1). In contrast to normally developed controls, clear endosperm developmental defects were observed in both reciprocal interploidy crosses. These results document strong yet still incomplete triploid block in A. arenosa. Despite this, the complete absence of adult triploid hybrids in the field suggests alternative mechanisms of cross-ploidy gene flow such as involvement of unreduced gametes and formation of one-step tetraploid hybrids.

Funding: Junior group leader research project of Charles University in Prague (Primus/SCI/35)

Key collaborators on this work are

Clement Lafon-Placette (Charles Univ. Prague)

Levi Yant, Kirsten Bomblies (JIC Norwich)

 

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