Interspecific evolution in plant microsatellite structure

SSR genotyping

SSR genotyping involves the use of simple sequence repeats (SSRs) as DNA markers. SSRs, also called microsatellites, are a type of repetitive DNA sequence ubiquitous in most plant genomes. SSRs contain repeats of a motif sequence 1-6 bp in length. Due to this structure SSRs frequently undergo mutations, mainly due to DNA polymerase errors, which involve the addition or subtraction of a repeat unit. Hence, SSR sequences are highly polymorphic and may be readily used for detection of allelic variation within populations. SSRs are present within both genic and nongenic regions and are occasionally transcribed, and hence may be identified in expressed sequence tags (ESTs) as well as more commonly in nongenic DNA sequences. SSR genotyping involves the design of DNA-based primers to amplify SSR sequences from extracted genomic DNA, followed by amplification of the SSR repeat region using polymerase chain reaction, and subsequent visualization of the resulting DNA products, usually using gel electrophoresis. These procedures are described in this chapter. SSRs have been one of the most favored molecular markers for plant genotyping in the last 20 years due to their high levels of polymorphism, wide distribution across most plant genomes, and ease of use and will continue to be a useful tool in many species for years to come.

Genome-wide analysis of microsatellite sequence in seven filamentous fungi

Abundance of microsatellites with repeated unit lengths of 1-6 base pairs in seven fungi: Aspergillus nidulans, Coprinus cinereus, Cryptococcus neoformans (serotype A), Fusarium graminearum, Magnaporthe grisea, Neurospora crassa and Ustilago maydis were investigated on genomic scale. The results showed that each species has its specific profile for different types and different motifs of SSR loci. Ascomycetes fungi M. grisea, N. crassa and basidiomycete fungus U. maydis adopt much more microsatellites than other fungi examined. Total amount of 15,751, 14,788 and 6,854 SSR loci were observed respectively, average density is 406, 389 and 347 per Mbp sequence; overall length of SSR sequence was 0.82%, 0.95% and 0.79% of genomic sequence respectively. While ascomycetes fungus F. graminearum and A. nidulans contains the least SSRs in the genomic DNA, only 4,679 and 4,837 tracts were observed in 36 Mb and 30 Mb genomic sequence respectively. Microsatellite repeats in protein coding regions are investigated in Aspergillus nidulans, Magnaporthe grisea, and Neurospora crassa also, the results show that the difference of different types and motifs among three fungi is very little than that in whole genomic sequence. For trinucleotide repeats, overrepresent (comparing to the total base pair of protein coding region) of AGC, GGC, AGG, ACG and ACC was observed in coding region, frequencies of AAC and AAG were not difference between coding and non-coding region, AAT, AGT and ATG were underrepresent in coding region excepted for A. nidulans, in which ATG was overrepresentative.

Genomic organization of the AODEF gene in Asparagus officinalis L

The perianths of Liliaceae plants, such as lily and tulip, have two whorls of almost identical petaloid organs, which are called tepals. According to the modified ABC model proposed in tulip, the class B genes are expressed in whorl 1 as well as whorls 2 and 3, so that the organs of whorls 1 and 2 have the same petaloid structure. The floral structure of asparagus (Asparagus officinalis L.) is similar to that of Liliaceae plants, however, the expression of B-class genes (AODEF, AOGLOA, AOGLOB) was not found in whorl 1, but was confined to whorls 2 and 3. This result does not support the modified ABC model in asparagus. In order to gain a better understanding of asparagus flower development, we have characterized a genomic clone of the AODEF gene. We compared the genomic organization and promoter sequence of AODEF with three well-studied DEF-like genes, DEFICIENS (Antirrhinum), APETALA3 (Arabidopsis), and OSMADS16 (rice). Exon-intron structures of these genes are well-conserved except for the large fifth intron in the AODEF gene and the OSMADS16 gene. Putative cis-elements including CArG-boxes were found in the promoter region and forty-two microsatellites were found in the AODEF genomic sequence.

Microsatellite analysis reveals a progressive widening of the genetic basis in the elite durum wheat germplasm

It has been argued that the level of genetic diversity in the modern durum wheat ( Triticum turgidum L. var. durum) elite germplasm may have declined due to the high selection pressure applied in breeding programs. In this study, 58 accessions covering a wide spectrum of genetic diversity of the cultivated durum wheat gene pool were characterized with 70 microsatellite loci (or simple sequence repeats, SSRs). On average, SSRs detected 5.6 different allelic variants per locus, with a mean diversity index (DI) equal to 0.56, thus revealing a diversity content comparable to those previously observed with SSRs in other small-grain cereal gene pools. The mean genetic similarity value was equal to 0.44. A highly diagnostic SSR set has been identified. A high variation in allele size was detected among SSR loci, suggesting a different suitability of these loci for estimating genetic diversity. The B genome was characterized by an overall polymorphism significantly higher than that of the A genome. Genetic diversity is organised in well-distinct sub-groups identified by the corresponding foundation-genotypes. A large portion (92.7%) of the molecular variation detected within the group of 45 modern cvs was accounted for by SSR alleles tracing back to ten foundation-genotypes; among those, the most recent CIMMYT-derived founders were genetically distant from the old Mediterranean ones. On the other hand, rare alleles were abundant, suggesting that a large number of genetic introgressions contributed to the foundation of the well-diversified germplasm herein considered. The profiles of recently released varieties indicate that the level of genetic diversity present in the modern durum wheat germplasm has actually increased over time.

DNA sequence variation in BpMADS2 gene in two populations of Betula pendula

The PISTILLATA (PI) homologue, BpMADS2, was isolated from silver birch (Betula pendula Roth) and used to study nucleotide polymorphism. Two regions (together about 2450 bp) comprising mainly untranslated sequences were sequenced from 10 individuals from each of two populations in Finland. The nucleotide polymorphism was low in the BpMADS2 locus, especially in the coding region. The synonymous site overall nucleotide diversity (pis) was 0.0043 and the nonsynonymous nucleotide diversity (pia) was only 0.000052. For the whole region, the pi values for the two populations were 0.0039 and 0.0045, and for the coding regions, the pi values were only 0 and 0.00066 (for the corresponding coding regions of Arabidopsis thaliana PI world-wide pi was 0.0021). Estimates of pi or theta did not differ significantly between the two populations, and the two populations were not diverged from each other. Two classes of BpMADS2 alleles were present in both populations, suggesting that this gene exhibits allelic dimorphism. In addition to the nucleotide site variation, two microsatellites were also associated within the haplotypes. This allelic dimorphism might be the result of postglacial re-colonization partly from northwestern, partly from southeastern/eastern refugia. The sequence comparison detected five recombination events in the regions studied. The large number of microsatellites in all of the three introns studied suggests that BpMADS2 is a hotspot for microsatellite formation.

Microsatellite analysis of a population crash and bottleneck in the Mauna Kea silversword, Argyroxiphium sandwicense ssp. sandwicense (Asteraceae), and its implications for reintroduction

The Mauna Kea silversword, Argyroxiphium sandwicense ssp. sandwicense, has experienced both a severe population crash associated with an increase in alien ungulate populations on Mauna Kea, and a population bottleneck associated with reintroduction. In this paper, we address the genetic consequences of both demographic events using eight microsatellite loci. The population crash was not accompanied by a significant reduction in number of alleles or heterozygosity. However, the population bottleneck was accompanied by significant reductions in observed number of alleles, effective number of alleles, and expected heterozygosity, though not in observed heterozygosity. The effective size of the population bottleneck was calculated using both observed heterozygosities and allele frequency variances. Both methods corroborated the historical census size of the population bottleneck of at most three individuals. The results suggest that: (i) small populations, even those that result from severe reductions in historical population size and extent, are not necessarily genetically depauperate; and (ii) species reintroduction plans need to be conceived and implemented carefully, with due consideration to the genetic impact of sampling for reintroduction.