Seeds of doubt: Mendel's choice of Hieracium to study inheritance, a case of right plant, wrong trait

Sex- versus apomixis-specific polymorphisms in the 5'UTR of APOLLO from Boechera shift gene expression from somatic to reproductive tissues in Arabidopsis

Introduction

Among candidate genes underlying the control components of apomixis, APOLLO is known for its strong linkage to apomeiosis in the genus Boechera. The gene has "apo alleles," which are characterized by a set of linked apomixis-specific polymorphisms, and "sex alleles." All apomictic Boechera genotypes are heterozygous for the apo/sex alleles, whereas all sexual genotypes are homozygous for sex alleles.

Methods

In this study, native and synthetic APOLLO promoters were characterized by detecting the expression level of the β-glucuronidase (GUS) gene in Arabidopsis.

Results

Comparing various flower developmental stages in transgenic lines containing different constructs with 2-kb native transgenic lines revealed that changes to the APOLLO promoter causes shifts in tissue and developmental stage specificity of GUS expression. Importantly, several apomixis-specific polymorphisms in the 5'UTR change the timing and location of GUS activity from somatic to reproductive tissues.

Discussion

These synthetic data simulate a plausible evolutionary process, whereby apomixis-specific gene activity can be achieved.

Genetic mapping of the LOSS OF PARTHENOGENESIS locus in Pilosella piloselloides and the evolution of apomixis in the Lactuceae

Pilosella piloselloides var. praealta (syn. P. praealta; Hieracium praealtum) is a versatile model used to study gametophytic apomixis. In this system apomixis is controlled by three loci: one that controls the avoidance of meiosis (LOA), one that controls the avoidance of fertilization (LOP) and a third that controls autonomous endosperm formation (AutE). Using a unique polyhaploid mapping approach the LOP locus was mapped to a 654 kb genomic interval syntenic to linkage group 8 of Lactuca sativa. Polyhaploids form through the gametophytic action of a dominant determinant at LOP, so the mapped region represents both a functional and a physical domain for LOP in P. piloselloides. Allele sequence divergence (ASD) analysis of the PARTHENOGENESIS (PAR) gene within the LOP locus revealed that dominant PAR alleles in Pilosella remain highly similar across the genus, whilst the recessive alleles are more divergent. A previous report noted that dominant PAR alleles in both Pilosella and Taraxacum are modified by the presence of a class II transposable element (TE) in the promoter of the gene. This observation was confirmed and further extended to the related genus Hieracium. Sufficient differences were noted in the structure and location of the TE elements to conclude that TE insertional events had occurred independently in the three genera. Measures of allele crossover amongst the polyhaploids revealed that P. piloselloides is an autopolyploid species with tetrasomic inheritance. It was also noted that the dominant determinant of LOP in P. piloselloides could transmit via a diploid gamete (pollen or egg) but not via a haploid gamete. Using this information, a model is presented of how gametophytic apomixis may have evolved in several members of the Lactuceae, a tribe of the Asteraceae.

Demystifying the mythical Mendel: a biographical review

Gregor Mendel is widely recognised as the founder of genetics. His experiments led him to devise an enduring theory, often distilled into what are now known as the principles of segregation and independent assortment. Although he clearly articulated these principles, his theory is considerably richer, encompassing the nature of fertilisation, the role of hybridisation in evolution, and aspects often considered as exceptions or extensions, such as pleiotropy, incomplete dominance, and epistasis. In an admirable attempt to formulate a more expansive theory, he researched hybridisation in at least twenty plant genera, intentionally choosing some species whose inheritance he knew would deviate from the patterns he observed in the garden pea (Pisum sativum). Regrettably, he published the results of only a few of these additional experiments; evidence of them is largely confined to letters he wrote to Carl von Nägeli. Because most original documentation is lost or destroyed, scholars have attempted to reconstruct his history and achievements from fragmentary evidence, a situation that has led to unfortunate omissions, errors, and speculations. These range from historical uncertainties, such as what motivated his experiments, to unfounded suppositions regarding his discoveries, including assertions that he never articulated the principles ascribed to him, staunchly opposed Darwinism, fictitiously recounted experiments, and falsified data to better accord with his theory. In this review, I have integrated historical and scientific evidence within a biographical framework to dispel misconceptions and provide a clearer and more complete view of who Mendel was and what he accomplished.

A PARTHENOGENESIS allele from apomictic dandelion can induce egg cell division without fertilization in lettuce

Apomixis, the clonal formation of seeds, is a rare yet widely distributed trait in flowering plants. We have isolated the PARTHENOGENESIS (PAR) gene from apomictic dandelion that triggers embryo development in unfertilized egg cells. PAR encodes a K2-2 zinc finger, EAR-domain protein. Unlike the recessive sexual alleles, the dominant PAR allele is expressed in egg cells and has a miniature inverted-repeat transposable element (MITE) transposon insertion in the promoter. The MITE-containing promoter can invoke a homologous gene from sexual lettuce to complement dandelion LOSS OF PARTHENOGENESIS mutants. A similar MITE is also present in the promoter of the PAR gene in apomictic forms of hawkweed, suggesting a case of parallel evolution. Heterologous expression of dandelion PAR in lettuce egg cells induced haploid embryo-like structures in the absence of fertilization. Sexual PAR alleles are expressed in pollen, suggesting that the gene product releases a block on embryogenesis after fertilization in sexual species while in apomictic species PAR expression triggers embryogenesis in the absence of fertilization.

Evolutionary history and genetic diversity of apomictic allopolyploids in Hieracium s.str.: morphological versus genomic features

PREMISE

The origin of allopolyploids is believed to shape their evolutionary potential, ecology, and geographical ranges. Morphologically distinct apomictic types sharing the same parental species belong to the most challenging groups of polyploids. We evaluated the origins and variation of two triploid taxa (Hieracium pallidiflorum, H. picroides) presumably derived from the same diploid parental pair (H. intybaceum, H. prenanthoides).

METHODS

We used a suite of approaches ranging from morphological, phylogenetic (three unlinked molecular markers), and cytogenetic analyses (in situ hybridization) to genome size screening and genome skimming.

RESULTS

Genotyping proved the expected parentage of all analyzed accessions of H. pallidiflorum and H. picroides and revealed that nearly all of them originated independently. Genome sizes and genome dosage largely corresponded to morphology, whereas the maternal origin of the allopolyploids had no discernable effect. Polyploid accessions of both parental species usually contained genetic material from other species. Given the phylogenetic distance of the parents, their chromosomes appeared only weakly differentiated in genomic in situ hybridization (GISH), as well as in overall comparisons of the repetitive fraction of their genomes. Furthermore, the repeatome of a phylogenetically more closely related species (H. umbellatum) differed significantly more.

CONCLUSIONS

We proved (1) multiple origins of hybridogeneous apomicts from the same diploid parental taxa, and (2) allopolyploid origins of polyploid accessions of the parental species. We also showed that the evolutionary dynamics of very fast evolving markers such as satellite DNA or transposable elements does not necessarily follow patterns of speciation.

The Boechera Genus as a Resource for Apomixis Research

The genera Boechera (A. Löve et D. Löve) and Arabidopsis, the latter containing the model plant Arabidopsis thaliana, belong to the same clade within the Brassicaceae family. Boechera is the only among the more than 370 genera in the Brassicaceae where apomixis is well documented. Apomixis refers to the asexual reproduction through seed, and a better understanding of the underlying mechanisms has great potential for applications in agriculture. The Boechera genus currently includes 110 species (of which 38 are reported to be triploid and thus apomictic), which are distributed mostly in the North America. The apomictic lineages of Boechera occur at both the diploid and triploid level and show signs of a hybridogenic origin, resulting in a modification of their chromosome structure, as reflected by alloploidy, aneuploidy, substitutions of homeologous chromosomes, and the presence of aberrant chromosomes. In this review, we discuss the advantages of the Boechera genus to study apomixis, consider its modes of reproduction as well as the inheritance and possible mechanisms controlling apomixis. We also consider population genetic aspects and a possible role of hybridization at the origin of apomixis in Boechera. The molecular tools available to study Boechera, such as transformation techniques, laser capture microdissection, analysis of transcriptomes etc. are also discussed. We survey available genome assemblies of Boechera spp. and point out the challenges to assemble the highly heterozygous genomes of apomictic species. Due to these challenges, we argue for the application of an alternative reference-free method for the comparative analysis of such genomes, provide an overview of genomic sequencing data in the genus Boechera suitable for such analysis, and provide examples of its application.

The Full Breadth of Mendel's Genetics

Gregor Mendel’s “Experiments on Plant Hybrids” (1865/1866), published 150 years ago, is without doubt one of the most brilliant works in biology. Curiously, Mendel’s later studies on Hieracium (hawkweed) are usually seen as a frustrating failure, because it is assumed that they were intended to confirm the segregation ratios he found in Pisum. Had this been his intention, such a confirmation would have failed, since, unknown to Mendel, Hieracium species mostly reproduce by means of clonal seeds (apomixis). Here we show that this assumption arises from a misunderstanding that could be explained by a missing page in Mendel’s first letter to Carl Nägeli. Mendel’s writings clearly indicate his interest in “constant hybrids,” hybrids which do not segregate, and which were “essentially different” from “variable hybrids” such as in Pisum. After the Pisum studies, Mendel worked mainly on Hieracium for 7 years where he found constant hybrids and some great surprises. He also continued to explore variable hybrids; both variable and constant hybrids were of interest to Mendel with respect to inheritance and to species evolution. Mendel considered that their similarities and differences might provide deep insights and that their differing behaviors were “individual manifestations of a higher more fundamental law.”