Genetic Hybridization barriers

The question of what is a species, though discussed since decades, has still not been fully answered. However, beyond controversy is the fact that in order for a new species to arise reproductive isolation must occur. Two different broad categories of reproductive isolation barriers have been classified acting either prezygotically or postzygotically. Whereas prezygotic barriers prevent fertilization to occur, postzygotic barriers act after fertilization and include hybrid inviability and sterility and are strongly correlated with the genetic distance between taxa. In the proposed research I aim investigating postzygotic hybridizations barriers that cause endosperm breakdown leading to hybrid seed abortion. The endosperm is a terminal nutritive tissue supporting embryo growth and it is essential for viable seed formation; therefore, malfunction of endosperm development will result in embryo arrest and seed abortion. My initial experiments revealed that there are strong postzygotic barriers in two different species pairs of the Brassica family: Capsella (C. rubella and C. grandiflora) and Arabidopsis (A. lyrata and A. arenosa), establishing these species pairs as model systems to investigate the basis for postzygotic hybridization barriers in plants. Specifically, the main aims of this proposal are to (i) morphologically characterize hybrid seed failure in Capsella rubella and C. grandiflora and Arabidopsis lyrata and A. arenosa, (ii)identify the underlying genetic basis of postzygotic hybridization barriers in Capsella, (iii) characterize the molecular mechanisms underlying hybrid seed failure in Capsella. I anticipate that the results of this research will strongly advance our understanding of the underlying molecular mechanisms of reproductive isolation and plant speciation and will be of interest to a broad scientific community.

Hybridization is one of the major forces leading to the development of new species and a driving force to evolution. Barriers to hybridization can restrict gene flow and establish reproductive isolation between different taxa. These barriers can either occur pre- or postzygotically before or after fertilization - and finally lead to full genetic isolation. Post-zygotic isolation can occur among recently diverged taxa and it has evolved rapidly in some species groups. The interactions of the newly joined parental genomes can lead to severe developmental problems in the hybrid, sterility and seed abortion. It has been found that these developmental problems can depend on the direction of the cross. If such a cross is conducted using one parent as either father or mother, respectively, the hybrid will show different developmental success. The aim of the project was to investigate this phenomenon in the genus Capsella. It included a detailed morphological description and an analysis to uncover the underlying genetic mechanisms. Two recently diverged species the self-pollinator Capsella rubella and the outcrosser C. grandiflora have been crossed reciprocally. While hybridization of C. rubella maternal plants with C. grandiflora pollen donors resulted in complete seed abortion the reciprocal cross showed formation of small seeds. 60 % of these small seeds developed into fully fertile hybrids. The cause of this seed abortion is due to developmental problems in the endosperm. The endosperm is an ephemeral tissue in the seed that nourishes the embryo during growth. It is essential for viable seed formation and mechanisms disrupting endosperm development will cause embryo arrest and seed abortion. Unlike the embryo that contains one chromosomal set of each parent, in most plant groups the endosperm is triploid it consists of three parental chromosomal sets two maternal and one paternal. If this chromosomal equilibrium is disturbed, meaning that hybridization occurs in a way that the inherited ratio of the parental chromosomes is not two maternal to one paternal, the development of the seed can be severely disrupted. Strikingly, the results of this project show, that the transcriptomic response of the hybrids, meaning the results of gene expression after hybridization, in both directions between C. rubella and C. grandiflora mimicked respectively the response of paternal and maternal excess hybridizations in the model plant Arabidopsis thaliana. This suggests that unbalanced genome dosage causes hybridization failure in both species. C. grandiflora behaved like a plant of increased chromosome numbers, called polyploid. It evoked a response that resembles an interploidy-type seed failure. That means that the qualitative differences in the parental genomes of those two Capsella species resemble quantitative differences of polyploid parental species that differ in their chromosome number. The data provides strong support for the theory that crosses between plants of different mating systems will be unbalanced. Given that both species in this project only recently diverged, it also suggests that a fast evolving mechanism underlies the post-zygotic hybridization barrier(s) separating both species.