Distyly- Genetics of Distyly in plants

Distyly- Genetics of Distyly;Evolution of distyly; Breakdown of heterostyly

1. Genetics of Distyly in plants Presented By Divya S- I Ph.D (GPB) 2019608004

2. Morphological characters - Distyly In distylous populations, each individual has flowers with a long style and short stamens (pin flowers) or flowers with a short style and long stamens (thrum flowers)

3. Association of distyly number of pollen grains produced per flower and pollen size The number of pollen grains produced per flower and pollen size polymorphism between the morphs are sometimes associated with heterostyly as ancillary characteristics. Pollen grains are produced more in the stamens of long-styled flowers than short-styled flowers in most distylous species that have been examined (Ganders 1979a; Dulberger 1992). The size of the pollen grains in the short-styled flowers is larger than in the long-styled flowers in some distylous plants (e.g. Baker 1956; Bir Bahadur 1968 )

4. Surface ornamentation of pollen grains- Distyly A difference in surface ornamentation in pollen grains has also been reported in some distylous species (Baker et al.,2003). Pollen grains are reticulate or spinulose in Waltheria (Köhler 1976), and smooth or granulate muri in Damnacanthus ( Naiki & Nagamasu 2003), in pollen grains of long-styled and short-styled flowers, respectively. Spinulose -Pollen grain Smooth pollen grain

5. Physiological characters and genetics This SI system is believed to be genetically controlled together with the three morphological characters: Anther height, Stigma height and pollen In most distylous species investigated, three tightly linked diallelic loci (G, P and A) are assumed to be responsible for the inheritance of distylous characters, making the supergene (S) G/g controls female characters including style length and incompatibility reaction at the stigma or in the style, P/p controls pollen size and its incompatibility reaction and A/a controls anther position.

6. Genetics behind the distyly Under this model, individuals with a GPA/ gpa genotype - (heterozygous) bear short-styled flowers, gpa / gpa genotype - (homozygous recessive) have long-styled flowers . In some distylous species, such as Hypericum aegypticum ( Ornduff 1979) and Armeria maritima var. maritima (Baker 1966), the inheritance pattern is reversed (GPA/ gpa : long styled, gpa / gpa : short-styled)

7. However, homostylous (i.e. the heights of the anther and stigma are the same) flowers occasionally appear by recombination within the supergene (Lewis & Jones 1992;Barrett & Shore2008). The inferred genotypes are gPA / gpa or gPA / gPA for long homostylous individuals, and Gpa / gpa or Gpa / Gpa for short homostylous individuals (Barrett & Shore 2008). Some distylous taxa, Amsinckia spectabilis (Ray et al 1979), Psychotria acuminate (Bawa&Beach1983), Hedyotis salzmanii ( Riveroset al. 1995) and Salvia brandegeei (Barrett et al. 2000), are self compatible. Aliciella heterostyla ( Tommer up & Porter 1998) shows both self-incompatibility and self-compatibility.

8. Models for evolution of distyly - Charlesworth and Charlesworth (1979) model Self-incompatibility first occurred within a homostylous population as a result of selection to prevent self-fertilization under conditions of pollinator limitation This was accompanied by stigma height polymorphism to reduce intra-flower interference between males and females, and subsequently reciprocal herkogamy evolved to promote legitimate pollination.

9. Lloyd and Webb (1992) Model They proposed that the ancestral flowers showed approach herkogamy, and subsequent stigma-height polymorphism followed by invading and spreading a mutant that had shortened style length, and then reciprocal herkogamy was established by the appearance of reverse herkogamous mutants

10. Gene responsible for style- dimorphism CYP734A50 is only present on the short-styled S-morph haplotype, it is specifically expressed in S-morph styles, and its loss or inactivation leads to long styles.  Huu et al . (2016)

11. Phylogenetic distribution of distyly Distyly is known from 26 families: Acanthaceae , Amaryllidaceae, Boraginaceae , Connaraceae , Erythroxylaceae , Fabaceae, Gelsemiaceae (formerly included in Loganiaceae), Gentianaceae, Hypericaceae , Iridaceae, Lamiaceae , Linaceae , Lythraceae , Malvaceae (including former Sterculiaceae ), Menyanthaceae , Oleaceae, Oxalidaceae , Passifloraceae (including former Turneraceae ), Plumbaginaceae , Polemoniaceae , Polygonaceae , Primulaceae , Rubiaceae , Santalaceae , Saxifragaceae and Schoepfiaceae (formerly included in Olacaceae ).

12. Phylogenetic distribution of distyly

13. Evolutionary breakdown of heterostyly Several evolutionary pathways explaining the breakdown of heterostyly to other sexual systems have been proposed (Charlesworth & Charlesworth 1979b; Beach & Bawa 1980; Barrett 1989). Genetically, heterostyly can sometimes collapse to homostyly by recombination within a heterostyly supergene. The occurrence of the recombinant individuals is thought to be an evolutionary pathway from distyly to secondary homostyly (Charlesworth & Charlesworth 1979b; Richards 1997). If the homostylous individuals have some advantages, they establish and spread in a population (Richards 1997).

14. Occurrence of heterostyly and ploidy level In many cases examined in this review (Table 2), distyly is also found in polyploid taxa, although some lineages have correlations between polyploidization and homostyly (or monomorphy ). Polyploidization itself does not necessarily appear to have direct effects on the breakdown of distyly . It is reasonable to postulate that a self compatible diploid might be aroused first by recombination and then polyploidization occurred from the recombinant. The pollen morphology of tetraploid taxa supports this evolutionary step. The homostylous self compatible tetraploid taxa, Primula concholoba and P. watsnii ( Tremayne & Richards 1993), and the monomorphic tetraploid taxa of Damnacanthus (Naiki&Nagamasu2003) have thrum-type pollen grains.

15. Contd., The self-compatibility in polyploids that originated from heterostylous plants might be formed or sometimes promoted by recombination in the diploid ancestors. Polyploid plants have, on average, higher self-fertilization rates than their diploid relatives in angiosperms (Barringer 2007). Heterostyly evolved from polyphyletic origins and there appears to be different genetic backgrounds among heterostylous taxa. More studies integrating chromosomal, breeding-system and phylogenetic analyses within genera and families are required to develop the evolutionary scenarios for breeding systems of heterostyly.

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