Introduction
The genus Origanum L. is currently considered to include 44 species within the tribe Mentheae Dumort. subfamily Nepetoideae (Dumort.) Luerss. (Ietswaart 1980, Harley et al. 2004). Gynodioecy is widespread in the genus and found in 15 species and six subspecies (Demyanova 1985, Godin and Demaynova 2013, Godin 2019, 2020). Major work on the taxonomy of the genus Origanum was carried out by J.H. Ietswaart (1980), who critically analyzed all taxa of the genus available at that time and assigned some of the previously independent species to six subspecies of Origanum vulgare L. The subspecies of this taxon differ in the shape and size of bracts and spicate thyrses. Moreover, Ietswaart reported perfect and pistillate flowers in all six subspecies of O. vulgare . Gynodioecy in O. vulgare(= O. vulgare ssp. vulgare ) has been known for a long time, first described by Charles Darwin (Darwin 1877). Subsequently, many traits of gynodioecy in this taxon have been studied, such as sex inheritance (Lewis and Crowe 1956, Jain 1968, Kheyr-Pour 1969), perianth size of perfect and pistillate flowers (Vereshchagina and Malanina 1974, Anisimova and Demyanova 2007, Godin and Evdokimova 2017a), and sex ratios in populations (Ietswaart et al. 1984, Trosenko 1994, Demyanova 2012, Godin and Evdokimova 2017b, Gordeeva and Komarevceva 2019). However, for the other five subspecies of O. vulgare , as well as for previously independent species, there are no data on gynodioecy. On the one hand, it can be assumed that the other four subspecies ofOriganum vulgare have the same traits as gynodioecy is quite widespread in the genus Origanum (Godin 2019, 2020). In members of the subfamily Nepetoideae there is a strong association between gynodioecy and ecological and biological traits such as the predominance of perennial herbaceous plants, a large number of species in the genera, their wide distribution in the temperate zone of the Northern Hemisphere, and the existence of three-celled pollen (Wunderlich 1967, Demyanova 1985, Godin and Demyanova 2013). Thus, we can assume that other species of the genus Origanum may also be marked by gynodioecy, but this has so far not been documented.
From the beginning of our studies to the present time, an explanation for the maintenance of females in populations of gynodioecious plants has remained elusive. Two main hypotheses have been postulated: benefit of reproductive compensation and obligate cross-pollination (Charlesworth 1999). According to the first hypothesis, females of many gynodioecious species have a higher seed set than hermaphrodites (Darwin 1877, Demyanova 1985). According to the second hypothesis, the fitness of females is higher than that of hermaphrodites, since the latter may experience inbreeding depression (Lloyd 1974).
In gynodioecious species, there is high variability in the frequency of females and hermaphrodites. Studying the changes in sex ratios can help to explain the processes of maintenance of gynodioecy, its stability, or evolutionary transformations taking place in the direction to dioecy (Demyanova 1985, Bailey and Delph 2007). Revealing the change in the sex ratios on the environmental gradient is especially informative since such dependencies indicate the factors of natural selection that affect the reproductive system of the species. As numerous studies have shown, the sex ratios in gynodioecious species depend on several factors: the average annual air temperature (Alonso and Herrera 2001), the degree of soil moisture (Demyanova and Ponomarev 1979), the level of vitality of the individuals (Delph 1990a), the degree of damage due to herbivory on hermaphrodites and females (Ashman and Penet 2007), and the population size (Caruso and Case 2007). For example, a positive correlation has been found between annual temperature and female frequency in populations of some species (Alonso and Herrera 2001, Vaughton and Ramsey 2004, Caruso and Case 2007), while a negative correlation in others (Puterbaugh et al. 1997, Asikainen and Mutikainen 2003). A negative correlation between annual precipitation and female frequency has been found in some species where the proportion of females was higher in arid areas (Costich 1995, Wolfe and Shmida 1997, Ashman 1999, Cuevas et al. 2005). On the other hand, many studies (e.g. Demyanova and Ponomarev 1979, Demyanova 2013) clearly showed that the frequency of females in gynodioecious Lamiaceae and Caryophyllaceae sharply decreased after severe droughts.
Females often outnumber hermaphrodites under conditions of environmental stress when resources are scarce (Sakai and Weller 1991, Barrett 1992, Costich 1995). This might be due to the unequal allocation of resources between hermaphrodites and females. Hermaphrodites produce both pollen and seeds, while females only use their resources to set seeds. This makes females less susceptible to resource constraints than hermaphrodites (Delph 1990b). Thus, environmental conditions can have a great influence on sex ratios in gynodioecious species.
This work aims to reveal the sexual polymorphism of flowers and determine sex ratios in populations of Origanum vulgare ssp.gracile (K. Koch) Ietsw. in Tajikistan.
Origanum vulgare ssp. gracile (= Origanum tyttanthum Gontsch.) is a perennial herbaceous short-rhizome polycarpic plant (Fig. 1), a hemicryptophyte. Its range covers eastern Turkey, northern parts of Iraq, Iran, Afghanistan, and Central Asia (Pamir-Altai, Western Tien Shan). It grows at an altitude of 800 to 2700 m above sea level in the middle mountain belt, among wood and scrub vegetation and in the subalpine zone, on fine earth and gravely soils (Borisova 1977).
In this study, we used flower morphology and sex ratios inOriganum vulgare ssp. gracile , to verify and further investigate the nature of this gynodioecious species, forming two types of flowers, perfect and pistillate (Fig. 2), on different individuals and to check whether the frequency of females depends on environmental variables.