Endosperm‐based postzygotic hybridization barriers: developmental mechanisms and evolutionary drivers

Abstract

The endosperm is a nourishing tissue that serves to support embryo growth. Failure of endosperm development will ultimately cause embryo arrest and seed lethality, a phenomenon that is frequently observed upon hybridization of related plant species or species that differ in ploidy. Endosperm-based interspecies or interploidy hybridization barriers depend on the direction of the hybridization, causing nonreciprocal seed defects. This reveals that the parental genomes are not equivalent, implicating parent-of-origin specific genes generating this type of hybridization barrier. Recent work revealed that endosperm-based hybridization barriers are rapidly evolving. In this review, we discuss the developmental mechanisms causing hybrid seed lethality in angiosperms as well as the evolutionary forces establishing endosperm-based postzygotic hybridization barriers.

Endosperm-based hybridization barriers explain the pattern of gene flow between Arabidopsis lyrata and Arabidopsis arenosa in Central Europe

“…As a consequence, interploidy hybrid seeds are usually inviable (29-31, 35, 36). Interestingly, very similar nonreciprocal seed phenotypes are also observed in interspecies hybrid seeds, even between species of same ploidy levels (36)(37)(38)(39). This finding suggests that species, despite having the same ploidy level, differ in their relative genome dosage, resulting in unbalanced contributions of cellularization factors leading to cellularization failure.…”

Is homoploid hybrid speciation that rare? An empiricist’s view

“…RI is a sine qua non condition to initiate speciation (Coyne and Orr, 2004) and intrinsic reproductive isolating mechanisms, in particular, maintain integrity of species whenever they come into contact. There is much theoretical and empirical research on the components of RI (Lafon-Placette and Köhler, 2016;Pease et al, 2016) and how to identify and measure them (Ramsey et al, 2003;Martin and Willis, 2007;Sobel and Chen, 2014). However, there is also a growing concern about the actual role of intrinsic reproductive isolating mechanisms in the speciation process itself, particularly in allopatric speciation (Wiens, 2004) and specifically on whether they are drivers or merely by-products of divergent evolution (Saetre, 2013).…”

Endosperm and Imprinting, Inextricably Linked

“…3; Scott et al, 1998;Pennington et al, 2008). The resemblance of reciprocal endosperm phenotypes produced in interploidy and interspecific crosses has led to the idea that these are functionally similar phenomenon, both possibly related to gene imprinting (Lin, 1984;Bushell et al, 2003;Lafon-Placette and Köhler, 2016). How might this be so?…”

“…Like ADM, SUVH7 and PEG2 imprinting is maintained in triploid seeds, but expression levels are higher. Together, these data have led to the suggestion that imprinted genes establish interploidy hybridization barriers, ultimately promoting speciation (Gutierrez-Marcos et al, 2003;Lafon-Placette and Köhler, 2016). Although it is clear that imprinted genes can impact this process, it is less clear that parentof-origin-specific expression is the relevant feature with regard to hybrid seed failure.…”