Autonomous apomixis in Praxelis clematidea (Asteraceae: Eupatorieae), an invasive alien plant

Background and Aims Invasion by alien species poses serious threats to ecosystem services, biodiversity, and economic development. Understanding the reproductive mechanisms of invasive alien species can lay the foundation for effective control measures. Praxelis clematidea R.M. King & H. Rob is a triploid neotropical Asteraceae species that is invasive in China and other countries. However, few studies have focused on its reproductive biology. Methods In this study, flow cytometric seed screening (FCSS) was used to identify and confirm the reproductive mode of the species. The development of ovules, anthers, and mega- and microgametophytes was observed using a clearing method and differential interference contrast microscopy. Pollen viability was measured using the Benzidine test and Alexander’s stain. Pollen morphology was observed via fluorescence microscopy after sectioning the disk florets and staining with water-soluble aniline blue or 4’6-diamidino-2-phenylindole nuclei dyes. Controlled pollination experiments were conducted on four populations in China to examine the breeding system and to confirm autonomous apomixis. Key Results The reproductive mode was found to be autonomous apomixis without pseudogamy, according to FCSS. Megaspore mother cells developed directly into eight-nucleate megagametophytes without meiosis, conforming to Antennaria-type diplospory. The unreduced egg cells developed into embryos through parthenogenesis, while the endosperm was formed by the fusion of two unreduced polar nuclei. Pollen viability was very low (0.82 ± 0.57% and 0.36 ± 0.44% as measured by the Benzidine test and Alexander’s stain, respectively). The majority of the pollen grains were empty and had neither cytoplasm nor nuclei. The seed set was >90% for all treatments of open-pollination, bagging, and emasculated capitula. Mature cypselae developed in capitula that were emasculated before flowering, which confirmed that the breeding system of P. clematidea was autonomous apomixis. Conclusions The present study is the first report of autonomous apomixis in P. clematidea in China. Antennaria-type autonomous apomixis in P. clematidea greatly increases the probability of successful colonisation and dispersal of P. clematidea into new areas, which likely contributes to its high invasion potential. Effective control measures should be implemented to prevent autonomous (pollen-independent) seed production.

Reproductive Biology in Northern Prickly Ash

Zanthoxylum americanum is a common understory species in the northern forests of Minnesota and surrounding regions. It has potential economic importance for its citrus fragrance, pharmacological or insecticidal properties, and produces peppercorns similar to those of the related Zanthoxylum species. Zanthoxylum americanum is a dioecious species but has been reported to have aberrant flowers with autonomous apomixis instead of other potential reproductive barriers. The reproductive biology of Zanthoxylum americanum was investigated in two native Minnesota populations. Determinations of male fertility, whether autonomous apomixis was the predominant floral reproductive mechanism, the presence of seedless fruit (parthenocarpy/stenospermocarpy), and the occurrence of hermaphrodism were made over 2 years. Sex ratios (female:male plants) within each population differed. The mean pollen stainability was 95.8% ± 0.3% (fresh) and 78.6% ± 1.1% (stored 18 months). Parthenocarpy did not occur in either population. Autonomous apomixis was not the primary floral reproductive mechanism. Stenospermocarpy (seedlessness) occurred in 13% of the female fruit clusters. Although commonly described as being dioecious, two additional reproductive strategies were identified: 1) plants with functional protandrous flowers with rudimentary pistils and 2) hermaphroditic flowers with fully functional pistils (protogynous) and anthers. As many as 10% to 30% of the male plants bore at least one fruit/plant each year. One clonal stand had both hermaphroditic and functionally staminate flowers on the same plant. Two evolutionary pathways to dioecy in Z. americanum are proposed.

Genomic imprinting in endosperm: its effect on seed development in crosses between species, and its implications for the evolution of apomixis

If a mother sometimes has offspring by more than one father and if genes in the offspring are active in acquiring resources from maternal tissues, theory predicts that alleles at some loci in the offspring will evolve different patterns of gene expression depending on the gene’s parent of origin (genomic imprinting). The criteria for the evolution of imprinting are satisfied in many seed plants, and imprinting has been reported from the endosperm of angiosperm seeds. This paper’s purpose is to show that imprinting phenomena in endosperm can provide a coherent explanation of some failures of experimental crosses, and of the prevalence of pseudogamy among apomictic angiosperms. As a consequence of imprinting, seed development comes to depend on a particular ratio of maternal and paternal genomes in endosperm. This ratio is normally two maternal genomes to each paternal genome. Imprinting probably accounts for the failure of crosses between diploids and their autotetraploids, because the 2m: 1p ratio is disturbed in such crosses. Imprinting may also account for the breakdown of endosperm in crosses between related species, if the expression of maternal and paternal genomes in endosperm is out of balance. When a cross fails because of such an imbalance, the reciprocal cross will have the opposite imbalance and a complementary phenotype would be expected. The embryological evidence is consistent with this prediction. For example, many incompatible crosses show delayed wall formation in one direction of the cross, but precocious wall formation in the other direction. Typically, seed development can be classified as showing ‘paternal excess’ or ‘maternal excess’. Paternal excess is associated with unusually vigorous early growth of the endosperm, and maternal excess with the opposite. This pattern is consistent with natural selection on paternal gene expression favouring larger seeds. Genetic evidence from maize confirms an association between paternal gene expression and larger kernel size, and maternal gene expression and smaller kernel size. Genomic imprinting creates a requirement for both maternal and paternal genomes in imprinted tissues. In mammals, imprinting is expressed in derivatives of the zygote. The requirement for a paternal genome has constituted a block to the evolution of parthenogenesis, because all the genes in a parthenogenetic embryo are maternal. In angiosperms, imprinting is primarily expressed in the endosperm rather than the embryo. If the effects of imprinting in the embryo are small, an asexually produced embryo can develop, provided that it is associated with a viable endosperm. Many a Pom^ ts are pseudogamous. That is, the endosperm is fertilized and contains maternal and paternal genes embryo is asexual and contains maternal genes only. Thus, the division of labour between the embryo the endosperm during development of the seed can be seen as a preadaptation for apomixis. Some apomicts are autonomous. That is, the embryo and the endosperm both develop without fertilization. Genomic imprinting in endosperm would seem to constitute a barrier to the evolution of autonomous apomixis. Thus, there is a problem, not previously appreciated, in understanding how autonomous apomixis is possible