The Pre-pollination Reproductive Isolating Mechanism Of Iris Domestica And I.Dichotoma

Sister species are the most closely related species,and the study of the reproductive isolating mechanism between sister species is particularly important for understanding the process of speciation.Iris dichotoma and I.domestica are sister species,and their hybrids were easily obtained by artificial pollination.However,no natural hybrid had been reported in nature.Also,we had planted these two species together side by side for many years,but had never found a natural hybrid.Therefore,we examined the pre-pollination reproductive isolating mechanism of these two sister species.In order to understand the geographic distribution of Iris dichotoma and I.domestica and to explore the role of geographical isolation in reproductive isolation,the geographic information was obtained by u sing the online specimen and image databases to map their distribution and predict the suitable areas,the environmental variables of different distribution locations were compared,the geographical isolation index was estimated,the sympatric condition of I.dichotoma and I.domestica was established to detected their natural hybrids.The field experiment system and the net house with honeybees experiment system were established under sympatric condition.A numerous of experiments were conducted under sympatric condition to analyze the role of temporal isolation and pollinator isolation in reproductive isolation,including detection of natural hybrids,comparison of daily flowering time,test of interspecific crossing,test of pollen transfer in the field,categorization and identification of flower-visiting insects,detection of period and frequency of insect visiting,daily dynamic changes of pollen quantity,test of pollen transfer in the net house with honeybees,and detection of positions of pollen deposited on the honeybee’s body.The results showed that I.dichotoma was mainly distributed in the northeastern and northern regions of China,while I.domestica was mainly distributed in the central and southern regions of China;The reproductive isolation index of I.dichotoma and I.domestica was close to 1;There were significant differences in the longitude,latitude,the average annual temperature,and the average annual rainfall of the distribution locations;No natural hybrids were found in 131 seedlings from 500open-pollinated seeds under sympatric condition;The daily flowering time of the two species was different,but overlapped;The overlapping time was 2.6 h.They have good hybrid compatibility during the overlapping period;The Apis ceranaa and the Western honeybee A.mellifer were common legitimate pollinators,but their visit time did not overlap,and when the I.dichotoma stared to open,the I.domestica had no pollen to transfer;The pollen quantity of I.domestica,which covered with insect-free net tent,showed that there was still remained until when the I.dichotoma started to open and the honeybees could visit the I.domestica and I.dichotoma at the same time,but due to the different positions of the pollen deposited on the honeybee’s body,it was difficult to find the pollen on the stigma of I.dichotoma.These results indicated that China is the origin of I.domestica and I.dichotoma,and temperature and precipitation may limit the dispersal of the two species;I.domestica and I.dichotoma were allopatric species with geographic isolation;I.domestica and I.dichotoma were still be isolated when eliminating the geographical isolation under sympatric condition;There were other reproductive isolating mechanisms in addition to geographical isolation;Although overlapping flowering times did not directly cause complete reproductive isolation,it led to non-overlapping honeybees’ visiting period,and lead to early depletion of I.domestica pollen quanity,resulting in reproductive isolation indirectly;The differences in floral structure also played an important role in reproductive isolation,but not caused reproductive isolation completely.This study demonstrated a new pattern of reproductive isolating mechanism.There were other pre-pollination reproductive isolating mechanisms in I.domestica and I.dichotoma in addition to geographic isolation.It provided an empirical evidence that allopatric sister species had other pre-zygotic reproductive isolation mechanisms.It provides an example for understanding the relative importance and order of pre-and postzygotic isolating mechanisms;supporting the notion that pre-zygotic reproductive isolating mechanisms can also be by-products of exotic species.In addition,the hybrid progeny of I.domestica and I.dichotoma,I.× norrisii,possesses numerous continuously variable traits,which can be used as a key plant material to study the process of origin and evolution of Iris.