Inter-genomic conflict in gynodioecy and its effects on molecular evolution of mitochondrial genomes

Lead Research Organisation: University of Edinburgh
Department Name: Sch of Biological Sciences

Abstract

Within species variation is an extremely important component of biodiversity to allow populations to adapt to changes in their environment. This is often related to environmental variation (e.g. north-south differences) or local environments (e.g. metal-tolerant plants growing on lead and copper mines, whereas others of the same species have no such tolerance). Here, we plan to study a case of variation that is maintained by natural selection acting through the benefits and costs of two different sex forms in a single plant species or population - hermaphrodites (which have both female and male functions, the situation in most plants) and females (or male steriles). In a few percent of flowering plants, both females and hermaphrodites co-occur. This is called gynodioecy.

The evolutionary processes involved in the maintenance of the sex forms can best be studied in natural populations with male sterile plants, such as many species in the genus Plantago (plantains). Plantago species are important components of wild grasslands, and easy to work with. Their genetics is quite well studied, and male steriles have been found in several species, making them the ideal study organisms.

Females are widely used in plant breeding, particularly in crops like maize where breeders want to produce hybrids, and also to prevent the 'escape' of pollen from genetically modified crops. There is thus much information about the inheritance of femaleness (male sterility) in plants. Male sterility is often caused by a mutation in the mitochondrial DNA of the plant (mitochondria are tiny structures in the cytoplasm of animal and plant cells that are essential for energy generation). This is called cytoplasmic male-sterility.

Cytoplasmic male sterility is a classic example of a 'selfish genetic element'. A species acquires a seemingly harmful mutation causing male sterility, or femaleness, despite the disadvantage compared to hermaphrodites due to loss of male fertility. This occurs because there are some advantages to being female - provided that pollen from hermaphrodites is available, females can often produce more seeds than the hermaphrodites, because, by 'selfishly' relying on others to fertilise their seeds, they have more resources available for seed production. Their offspring also often have higher survival, because females always mate with a different individual (hermaphrodites often reproduce by self-fertilisation and these progeny often have low survival or fertility, called 'inbreeding depression').

Sometimes the sterility and non-sterility variants can both remain in a population, and hermaphrodite and female plants may coexist for a long time, with mitochondrial DNA variation within the species. However, mutations in the nuclear DNA can restore the lost female function, leading to hermaphroditism even when the mitochondria are mutant. There is thus a conflict between nuclear and mitochondrial genes, rather like that in an influenza epidemic, where a new virus appears through mutation, and resistance against it builds up in the population until a new virus outbreak, of a different type, occurs (in this situation, the host's resistance is due to immune system changes, not to resistance mutations spreading in the host population). In the case of male sterility, a nuclear restorer mutation can sometimes spread in a plant population, making the plants mostly hermaphrodite again. The sterility mitochondrial type's advantages explained above cause this type to then be the only one remaining. If a new male sterility mutation later invades the species, the process can be repeated. Another interesting fact is that the mitochondrial DNA of Plantago evolves thousands of times faster than in most other plants, and we will also investigate a possible connection between this fast evolution and the different sex types. The results of the project will increase our understanding of the processes involved in the maintenance of male sterility.

Planned Impact

Biological scientists will be the greatest beneficiaries of this research project. The results will also have societal and economic value, as they will provide scientific understanding and underpinning of methods widely used in plant breeding and sustainable agriculture. Most closely related to the specific research goals of the project is that male sterility is important in breeding a wide range of crops, to generate hybrid seed and to prevent pollen escape in transgenic crops.

The research will contribute to human health and wealth by informing policy makers about potential side-effects of genetic factors used in plant breeding, and paternal inheritance of cytoplasmic DNA and its consequences for transfer of genes via pollen, which is important for protection of wild species related to genetically modified crop plants. Crop breeders hope to develop methods to induce mitochondrial rearrangements that cause male sterility (Sandhu et al 2007, PNAS USA 104: 1766). To benefit from such techniques, an understanding of naturally occurring male sterility polymorphisms is important, because such studies allow detection of weak selection, significant over multiple generations, but often too small to detect experimentally. If new cytoplasmic male sterility (CMS) mutations can readily evolve (as in the 'epidemic model' to be tested by our project) this would suggest that deleterious side-effects, even of small magnitude, are not of major concern. However, it may also suggest that restoration can readily evolve, so that such genotypes may be useful for only a limited time. Another important practical issue is whether (and how much) restorer genes lower plant performance. If joint cytoplasmic-nuclear systems, with nuclear restorer genes, are maintained in the long term in natural male-sterile systems, detrimental effects of restorers ('cost of restoration') is necessary. The development of markers to monitor frequencies of the underlying genetic factors in gynodioecious populations will be very useful for future tests in natural populations.

The general public are readily interested in plant diversity, and variation in sex expression is a particularly interesting topic. Amateur botanists have some understanding of diversity and population processes, which can be enriched, and associations such as the Botanical Society of the British Isles and Ecological Society can be very helpful in field parts of projects such as ours. Further, it is essential to cultivate a deeper interest in plants among children: plant reproduction and food security offer excellent opportunities to reach teachers and children in an accessible way and build on their prior knowledge. The Newcastle Open Air Laboratory organises popular workshops for schoolchildren, and we will use their expertise (see below).

The research contributes to NERC's recently developed Science Themes, in particular the Biodiversity theme, and objectives for training and career development of researchers. Specifically, it will help build the capacity of the UK research base, particularly in Biodiversity and Environmental Biology. The approaches used, including natural sequence variants, are increasingly important in genetic mapping, as is the use of population genetics approaches to test selection. The project includes multiple elements of training and development of early career researchers: it develops molecular, field and communication skills of two postdoctoral researchers and two technicians, thereby contributing to the building of UK research capacity in the area of plant population genetics and molecular taxonomy, skills which can be applied across the life sciences, in applied as well as basic research.

Publications

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Description We discovered that relationships between Plantago species were not as expected based on the published literature (which did not quantify relationships, but merely showed phylogenetic trees), and that species that we had hoped were close relatives (needed for use in our tests) were in fact distantly related. This meant that we had to find other species for our tests. When the sequence data were finally assembled, the results of our analysis suggested that the focal species does not have unexpectedly high sequence in its mitochondrial genome. In turn, this suggests that there has not been long-term maintenance of different mitochondrial genotypes associated with the male sterility observed in this species. Our study species therefore differs from the results for Silene vulgaris, the only species with male sterility that has previously been studied in this manner.

We also found that the mitochondrial sequences of several genes (haplotypes) in Plantago lanceolata (ribwort plantain) fall into 2 sets, one set with sequences similar to those in related species, and one set with many differences in the sequences. The former mitochondrial type is associated with not being male-sterile. This is the first case in which naturally occurring male sterility has been associated with mitochondrial sequence variants. Our finding that multiple genes differ between the two sex types is puzzling, and we are doing analyses to explore possible causes. In addition, the mitochondrial genome of this species includes variants with rearranged orders of the genes, and deletions of parts of the genome.

We also collaborated in a study of mitochondrial heteroplasmy (where individuals have more than a single mitochondrial genome sequence) in this plant. Heteroplasmy was frequently found in the wild, and this indicates that mitochondria can sometimes be transmitted by paternal plants to their offspring, i.e. that transmission is not strictly maternal. Also uncovered evidence for the soft selective sweep model of cytonuclear male sterility.
Exploitation Route Future research will build on these findings and the approaches we used to obtain them, including using the genetic variants we have discovered. For example, it will become possible to study heteroplasmy in more detail.

Our work uncovered the first case in which naturally occurring male sterility has been associated with mitochondrial sequence variants. This will help others working on such species to plan new studies, now that our approach has been shown to be helpful. Our finding that mitochondrial genome sequences within a single species contain deletion and rearrangement variants is also important information for future work on plant mitochondrial genomes.
Sectors Agriculture, Food and Drink,Other

 
Description iNTER-GENOMIC CONFLICT IN GYNODIOECY AND ITS EFFECTS ON MOLECULAR EVOLUTION OF MITOCHONDRIAL GENOMES 
Organisation Newcastle University
Country United Kingdom 
Sector Academic/University 
PI Contribution Newcastle are sequencing mitochondrial genes, and Edinburgh nuclear genes. Edinburgh will analyse the results to test whether the mitochondrial genes have unexpectedly high sequence diversity.
Collaborator Contribution Sharing sequencing effort, choosing suitable species, sharing data for a joint analysis.
Impact Data are accumulating
Start Year 2009