Intertribal hybrid plants produced from crossing Arabidopsis thaliana with apomictic Boechera.

Arabidopsis thaliana and Boechera belong to different tribes of the Brassicaceae and last shared a common ancestor 13-35 million years ago. A. thaliana reproduces sexually but some Boechera accessions reproduce by apomixis (asexual reproduction by seed). The two species are reproductively isolated, preventing introgression of the trait(s) controlling apomixis from Boechera into A. thaliana and their molecular characterisation. To identify if "escapers" from such hybridisation barriers exist, we crossed diploid or tetraploid A. thaliana mothers carrying a conditional male sterile mutation with a triploid Boechera apomict. These cross-pollinations generated zygotes and embryos. Most aborted or suffered multiple developmental defects at all stages of growth, but some seed matured and germinated. Seedlings grew slowly but eventually some developed into mature plants that were novel synthetic allopolyploid hybrids. With one exception, intertribal hybrids contained three Boechera plus either one or two A. thaliana genomes (depending on maternal ploidy) and were male and female sterile. The exception was a semi-fertile, sexual partial hybrid with one Boechera plus two A. thaliana genomes. The synthesis of "escapers" that survive rigorous early developmental challenges in crosses between A. thaliana and Boechera demonstrates that the inviability form of postzygotic reproductive isolation separating these distantly related species is not impenetrable. The recovery of a single semi-fertile partial hybrid also demonstrates that hybrid sterility, another form of postzygotic reproductive isolation, can be overcome between these species.

Does Father Know Best? A Formal Model of the Paternal Influence on Childhood Social Anxiety.

We explore paternal social anxiety as a specific risk factor for childhood social anxiety in a rational optimization model. In the course of human evolution, fathers specialized in external protection (e.g., confronting the external world) while mothers specialized in internal protection (e.g., providing comfort and food). Thus, children may instinctively be more influenced by the information signaled by paternal versus maternal behavior with respect to potential external threats. As a result, if fathers exhibit social anxiety, children interpret it as a strong negative signal about the external social world and rationally adjust their beliefs, thus becoming stressed. Under the assumption that paternal signals on social threats are more influential, a rational cognitive inference leads children of socially anxious fathers to develop social anxiety, unlike children of socially anxious mothers. We show in the model that mothers cannot easily compensate for anxious paternal behavior, but choose to increase maternal care to maintain the child's wellbeing. We discuss research directions to test the proposed model as well as implications for the prevention and treatment of child social anxiety.

Sexual and apomictic plant reproduction in the genomics era: exploring the mechanisms potentially useful in crop plants.

Arabidopsis, Mimulus and tomato have emerged as model plants in researching genetic and molecular basis of differences in mating systems. Variations in floral traits and loss of self-incompatibility have been associated with mating system differences in crops. Genomics research has advanced considerably, both in model and crop plants, which may provide opportunities to modify breeding systems as evidenced in Arabidopsis and tomato. Mating system, however, not recombination per se, has greater effect on the level of polymorphism. Generating targeted recombination remains one of the most important factors for crop genetic enhancement. Asexual reproduction through seeds or apomixis, by producing maternal clones, presents a tremendous potential for agriculture. Although believed to be under simple genetic control, recent research has revealed that apomixis results as a consequence of the deregulation of the timing of sexual events rather than being the product of specific apomixis genes. Further, forward genetic studies in Arabidopsis have permitted the isolation of novel genes reported to control meiosis I and II entry. Mutations in these genes trigger the production of unreduced or apomeiotic megagametes and are an important step toward understanding and engineering apomixis.

Description of a fertilization-independent obligate apomictic species: Corunastylis apostasioides Fitzg.

The Australian midge orchid Corunastylis apostasioides of the tribe Diurideae has completely eliminated any male contribution in the process of seed formation, which occurs directly from the maternal tissue by a process termed apomixis. Here, we report C. apostasioides to be an obligate apomictic species devoid of any sexuality and compare its development to a close sexual relative C. fimbriata (R. Br.) D.L. Jones & M.A. Clem. Apomictic characteristics in C. apostasioides include production of seed in absence of fertilization, frequently closed flowers, production of immature pollen in non-dehiscent anthers, expansion of ovaries despite the lack of fertilization and the absence of a citronella scent that is found in C. fimbriata produced to attract pollinating vinegar flies (Jones 2006). The nature of apomixis in C. apostasioides was examined by ovule histology and amplified fragment length polymorphism (AFLP) in each case drawing comparison with sexual C. fimbriata. In C. apostasioides the central megaspore mother cell undergoes diplosporic apomixis, while additional embryos are derived from nucellar or integument initials formed by sporophytic apomixis. Typical of apomicts, C. apostasioides is polyploid compared to the sexual C. fimbriata. The divergences of C. apostasioides from sexuality to apomictic development are discussed.

Seed development and inheritance studies in apomictic maize-Tripsacum hybrids reveal barriers for the transfer of apomixis into sexual crops.

Apomixis in plants covers a variety of cloning systems through seeds of great potential for plant breeding. Among long-standing approaches for crop improvement is the attempt to exploit wild relatives as natural, vast reservoirs for novel genetic variation. With regard to apomixis, maize possesses an apomictic wild relative, Tripsacum, which we used to produce advanced maize-Tripsacum hybrid generations. However, introgression of apomixis in maize has failed so far. In order to understand the hows and whys, we undertook characterization of seed development and inheritance studies in these materials. We show that apomictic seeds suffer from epigenetic loads. Both seed tissues, the endosperm and the embryo, displayed developmental defects resulting from imbalanced parental genomic contributions and aberrant methylation patterns, respectively. Progeny characterization of several maize-Tripsacum hybrid generations allowed significant progress toward the unraveling of the genetics of apomixis. First, chromosome deletion mapping showed that expression of apomixis requires one single Tripsacum chromosome. However, inheritance studies revealed that female gametes inheriting this segment were unequivalent carriers depending on their origin: unreduced gametes transmit a functional segment, whereas progeny derived from reduced ones reproduced sexually. Finally, chromosomal or genomic dosage variation barely affected the apomictic phenotype suggesting no dependency for ploidy in these materials. We conclude that epigenetic information imposes constraints for apomictic seed development and seems pivotal for transgenerational propagation of apomixis. The nature of the triggering mechanisms remains unknown as-yet, but it certainly explains the modest success relative to the development of apomictic maize thus far.

Towards molecular breeding of reproductive traits in cereal crops.

The transition from vegetative to reproductive phase, flowering per se, floral organ development, panicle structure and morphology, meiosis, pollination and fertilization, cytoplasmic male sterility (CMS) and fertility restoration, and grain development are the main reproductive traits. Unlocking their genetic insights will enable plant breeders to manipulate these traits in cereal germplasm enhancement. Multiple genes or quantitative trait loci (QTLs) affecting flowering (phase transition, photoperiod and vernalization, flowering per se), panicle morphology and grain development have been cloned, and gene expression research has provided new information about the nature of complex genetic networks involved in the expression of these traits. Molecular biology is also facilitating the identification of diverse CMS sources in hybrid breeding. Few Rf (fertility restorer) genes have been cloned in maize, rice and sorghum. DNA markers are now used to assess the genetic purity of hybrids and their parental lines, and to pyramid Rf or tms (thermosensitive male sterility) genes in rice. Transgene(s) can be used to create de novo CMS trait in cereals. The understanding of reproductive biology facilitated by functional genomics will allow a better manipulation of genes by crop breeders and their potential use across species through genetic transformation.

Timing of the maternal-to-zygotic transition during early seed development in maize.

In animals, early embryonic development is largely dependent on maternal transcripts synthesized during gametogenesis. Recent data in plants also suggest maternal control over early seed development, but the actual timing of zygotic genome activation is unclear. Here, we analyzed the timing of the maternal-to-zygotic transition during early Zea mays seed development. We show that for 16 genes expressed during early seed development, only maternally inherited alleles are detected during 3 d after fertilization in both the embryo and the endosperm. Microarray analyses of precocious embryonic development in apomictic hybrids between maize and its wild relative, Tripsacum, demonstrate that early embryo development occurs without significant quantitative changes to the transcript population in the ovule before fertilization. Precocious embryo development is also correlated with a higher proportion of polyadenylated mRNA in the ovules. Our data suggest that the maternal-to-zygotic transition occurs several days after fertilization. By contrast, novel transcription accompanies early endosperm development, indicating that different mechanisms are involved in the initiation of endosperm and embryo development.

Heterochronic expression of sexual reproductive programs during apomictic development in Tripsacum.

Some angiosperms reproduce by apomixis, a natural way of cloning through seeds. Apomictic plants bypass both meiosis and egg cell fertilization, producing progeny that are genetic replicas of the mother plant. In this report, we analyze reproductive development in Tripsacum dactyloides, an apomictic relative of maize, and in experimental apomictic hybrids between maize and Tripsacum. We show that apomictic reproduction is characterized by an alteration of developmental timing of both sporogenesis and early embryo development. The absence of female meiosis in apomictic Tripsacum results from an early termination of female meiosis. Similarly, parthenogenetic development of a maternal embryo in apomicts results from precocious induction of early embryogenesis events. We also show that male meiosis in apomicts is characterized by comparable asynchronous expression of developmental stages. Apomixis thus results in an array of possible phenotypes, including wild-type sexual development. Overall, our observations suggest that apomixis in Tripsacum is a heterochronic phenotype; i.e., it relies on a deregulation of the timing of reproductive events, rather than on the alteration of a specific component of the reproductive pathway.