Megaspore competition in F1 and F2 hybrids between Oenothera hookeri and Oe. suaveolens

Megasporogenesis and development of the embryo sac were investigated in F₁ and F₂ hybrids from crosses of <em>Oe. hookeri</em> and <em>Oe. suaveolens</em>. All hybrids form heteropolar and homopolar magaspore tetrads; the embryo sac, however, usually develops from the micropylar megaspore. Its development may occur immediately after degeneration of three other megaspores or after a period of competition between both apical megaspores. They develop simultaneously for a relatively short time, after which the growth of the chalazal megaspore is inhibited, although the latter does not degenerate. The micropylar megaspore as a rule develops without disturbances into the embryo sac, but in some ovules it is formed from the chalazal megaspore or double ones arise from both apical megaspores of the tetrad. The frequency of the micropylar embryo sac formation seems to be dependent above all on the hybrid plant genome and not on the haploid genome of the megaspore.

Pollen tube branching in the ovary of five species of Oenothera

Flowers of <em>Oenothera hookeri</em> Torr. et Gray, <em>Oe. brevistylis</em> and <em>Oe. lamarkiana</em> de Vries were pollinated after anthesis by insects. <em>Oe. biennis</em> L., <em>Oe. suaveolens</em> Desf and sulfurea were selfpollinated in the buds. Pollen morphology was slightly different: <em>Oe. hookeri</em> have regular, triporated pollen, often germinating through two pores; in <em>Oe. suaveolens</em> many pollen grains had callose patches on the intine; in <em>Oe. brevistylis</em> tetraporated pollen were more often than in other species; in <em>Oe. lamarckiana</em> many pollen grains were empty; in <em>Oe. biennis</em> and <em>Oe. suaveolens</em> pollen grain size and viability varied. The pollen tube growth and fertilization were similar in 5 species and can be considered as typical for <em>Oenothera</em>. In the ovary pollen tubes branched and changed their growth direction. Near micropyle they formed short branches to the inner integument. In the nucellus the pollen tube became swollen.