Evidence of natural hybridization and introgression between Medicago ciliaris and Medicago intertexta
Abstract
The present study, investigated some reproductive and fertility parameters in some wild populations , originating from the North Tunisia (4 populations of Medicago ciliaris and 3 populations of Medicago intertexta). Previous finding revealed that these species are genetically distinct and easily recognized by the number of flowers per inflorescence and pod dimensions. However, biometrical traits and isozyme patterns intermediacy between these two species had detected the existence of a potential spontaneous interspecific hybrid originating from Sedjnane locality in Tunisia. Indeed, the present work has sh own significant decrease of pollen fertility and seed production for this population when compared to the others (pollen viability 75%, pollen germinability 8% and pod production=9%). These results suggested a possible natural interspecific hybrid and confirming introgressive hybridization possibility between M. intertexta and M. ciliaris.
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Introduction
For a given species, the reproductive mode is a critically important factor influencing the genetic structure of the natural populations and allows to explain polymorphism. The breeding system affects also many aspects of crop management [1] and contributes to specify the mechanisms of infraspecific differentiation and consequently, the taxonomic value of many forms described within some complex of species [2,3].
Pollen study can be used to differentiate between different species taxonomy and identify possible interspecific hybrids [4,5]. Male sterility derived from irregularity, at any stage, since microsporogenesis to pollen maturation [6]. Pollen abortion of male sterility is an essential requirement in hybrid breeding. This system is verified by several features: Pollen can be characterized on the basis of nuclei number or on the basis of shape and staining behavior [7].The low pollen fertility of the hybrids may be due to chromosome disturbance caused by incompatibility barriers [8].
The genus Medicago contains perennial species essentially tetraploid cross-pollinated and annual species largely diploid with a predominant autogamous reproduction mode [9].These annual species are possibly useful to farmers for the production of organic nitrogen and prevention of soil erosion, and also pasture through periods of low production from traditional pasture species [10].
In previous studies, genetic diversity of some natural populations of Medicago ciliaris and Medicago intertexta (2n=16) originating from Tunisia has been analyzed based on biometrical and isozyme characters. The results have revealed that these species are genetically distinct and easily recognized by the number of flowers per inflorescence and pod dimensions 11]. However, morphological traits and isozyme patterns intermediacy between these two species has been demonstrated in one population originating from Sedjnane region [11]. This population was regarded as being a potential natural interspecific hybrid between the two species.
Characterization of interspecific hybridization in plants has been revealed in various studies [10,12,13,14]. Several methods have been used to reveal hybridity, including intermediate morphology [15,16,17], in situ cross hybridization [18], isozyme [19,20] and DNA [21,22]. The detection of decreased pollen viability or pollen fertility in plant is usually used besides molecular markers for the identification of spontaneous interspecific hybrids [23,24,25].
Pollen viability study is also commonly used in plant breeding for the measure of the reproductive ability of hybrids [24,26]. The aims of this work were to confirm the potential natural hybrid collected from Sedjnane locality. Therefore, fertility comparison study was carried out on several wild populations of Medicago ciliaris and Medicago intertexta based on pollen viability and germinability and also other aspects of sexual reproduction fitness (pod production, number of seeds per pod and percentage of ovules fertilized and transformed in seeds.
Conclusion
Fertility comparisons of the analyzed populations revealed high variability in reproduction ability. Whereas the hybrid's behaviour of Sedjnane area reacted like an interspecific hybrid between the two species, its fertility was dramatically disturbed. Therefore, individuals of this population observed in the field should be avoided in breeding programs because it may contribute to improve crop breeding and seed propagation effects. Further studies are required, particularly in pollen physiology and cytogenetic investigations associated with observation of fertility problems, to enhance our understanding about the observed natural hybrid. Moreover, development of molecular markers can be used to confirm the genomic contribution of parental taxa to suspected hybrid and its potential implication in the taxonomic problems within the Medicago genus.
