Genetics of red clover (Trifolium pratense L.), compatibility. I

Limited genetic changes observed during in situ and ex situ conservation in Nordic populations of red clover (Trifolium pratense)

Introduction

In situ and ex situ conservation are the two main approaches for preserving genetic diversity. The advantages and disadvantages of the two approaches have been discussed but their genetic effects have not been fully evaluated.

Methods

In this study we investigate the effects of the two conservation approaches on genetic diversity in red clover. Seed samples collected from wild populations in Sweden and Norway in 1980, their subsequent generations created during seed regeneration at the gene bank and samples recollected from the same location as the original samples, were analyzed with microsatellite markers, alongside reference samples from cultivars.

Results

Overall, there was a differentiation between cultivars and the wild material and between wild material from Sweden and Norway. In general, the original collections clustered together with the later generations of the same accession in the gene bank, and with the recollected samples from the same location, and the level of diversity remained the same among samples of the same accession. Limited gene flow from cultivated varieties to the wild populations was detected; however, some wild individuals are likely to be escapees or affected by gene flow.

Discussion

In conclusion, there were examples of genetic changes within individual accessions both in situ and ex situ, as is also to be expected in any living population. However, we observed only limited genetic changes in both in situ and ex situ conservation over the generations included in this study and with the relatively large populations used in the ex situ conservation in the gene bank at NordGen.

Late-acting self-incompatibility--the pariah breeding system in flowering plants

It is estimated that around half of all species of flowering plants show self-incompatibility (SI). However, the great majority of species alleged to have SI simply comply with 'the inability of a fully fertile hermaphrodite plant to produce zygotes when self-pollinated'--a definition that is neutral as to cause. Surprisingly few species have been investigated experimentally to determine whether their SI has the type of genetic control found in one of the three established mechanisms, that is, homomorphic gametophytic, homomorphic sporophytic or heteromorphic SI. Furthermore, our knowledge of the molecular basis of homomorphic SI derives from a few species in just five families--a small sample that has nevertheless revealed the existence of three different molecular mechanisms. Importantly, a sizeable cohort of species are self-sterile despite the fact that self-pollen tubes reach the ovary and in most cases penetrate ovules, a phenomenon called late-acting self-incompatibility (LSI). This review draws attention to the confusion between species that show 'self-incompatibility' and those that possess one of the 'conventional SI mechanisms' and to argue the case for recognition of LSI as having a widespread occurrence and as a mechanism that inhibits selfing and promotes outbreeding in many plant species.

Genetic map-based location of the red clover (Trifolium pratense L.) gametophytic self-incompatibility locus

Red clover is a hermaphroditic allogamous diploid (2n = 2x = 14) with a homomorphic gametophytic self-incompatibility (GSI) system (Trifolium pratense L.). Red clover GSI has long been studied, and it is thought that the genetic control of GSI constitutes a single locus. Although GSI genes have been identified in other species, the genomic location of the red clover GSI-locus remains unknown. The objective of this study was to use a mapping-based approach to identify simple sequence repeats (SSR) that were closely linked to the GSI-locus. Previously published SSR markers were used in this effort (Sato et al. in DNA Res 12:301-364, 2005). A bi-parental cross was initiated in which the parents were known to have one self-incompatibility allele (S-allele) in common. S-allele genotypes of 100 progeny were determined through test crosses and pollen compatibility. Pseudo F(1) linkage analysis isolated the GSI-locus on red clover linkage-group one within 2.5 cM of markers RCS5615, RCS0810, and RCS3161. A second 256 progeny mapping testcross population of a heterozygous self-compatible mutant revealed that this specific self-compatible mutant mapped to the same location as the GSI-locus. Finally, 82 genotypes were identified whose parents putatively shared one S-allele in common from maternal halfsib families derived from two random mating populations in which paternal identity was determined using molecular markers. Unique S-allele identity in the two random mating populations was tentatively inferred based on haplotypes of two highly allelic linkage-group one SSR (RCS0810 and RCS4956), which were closely linked to each other and the GSI-locus. Paternally derived pollen haplotype linkage analysis of RCS0810 and RCS4956 SSR and the GSI-locus again revealed tight linkage at 2.5 and 4.7 cM between the GSI-locus and RCS0810 and RCS4956, respectively. The map-based location of the GSI-locus in red clover has many immediate applications to red clover plant breeding and could be useful in helping to sequence the GSI-locus.

The inheritance of pseudo-self-compatibility (PSC) in Nemesia strumosa Benth

Pseudo-self-compatibility (PSC) in Nemesia strumosa was determined by recessive modifying genes which interferred with the normal stylar incompatibility reaction. The PSC levels ranged from none or low to amounts resembling full self-compatibility. PSC within a progeny could be maintained at low or high levels by selecting parents either high or low in PSC. Pollen from plants of any PSC level failed to produce seed when placed on other incompatible styles bearing the same S alleles.

OVULE DEVELOPMENT IN DIPLOID RED CLOVER

In red clover only one ovule of the two in each ovary regularly develops into a seed. Results of two experiments designed to reveal the fate of the ovules initially present provide a basis for comparing some features of ovule development in high- and low-seed-yielding plants. High-yielding plants produced embryo sacs in over 70% of ovules whereas low-yielding ones produced them in about 40%. The ability to produce embryo sacs seems to be genetically determined but strongly influenced by environment and occasionally by meiotic irregularities. Development started in about 85% of the fertile ovules in the high-yielding plants but in only about 60% of them in low-yielding plants. Apparently there was slow or limited growth of pollen tubes in the latter plants. Nearly 55% of the ovules developing just after fertilization matured into seeds in the high-yielding plants but only about 25% in the low ones. The higher frequency of ovule collapse in low-yielding plants seems to be a consequence of the delayed fertilization followed by retarded growth of the embryos and endosperms.Although a few more ovules did start to develop in out-crosses than in sib-crosses, the more vigorous growth of the endosperm seems quite important in accounting for the superior seed yields in the out-crosses. Also a few more ovules started to develop when the pollen came from a high-yielding plant instead of a low-yielding one and this initial advantage seems to lead to improved seed production.It is suggested that low-seed-producing plants may have certain genes that act like weak self-incompatibility alleles by inhibiting pollen tube growth, or, that their genetic constitutions are such that they cannot produce enough embryo sacs and/or support the growth of pollen tubes and ovules.