Ambiguous species boundaries: Hybridization and morphological variation in two closely related Rubus species along altitudinal gradients

The puzzle of plant hybridisation: a high propensity to hybridise but few hybrid zones reported

An interesting conundrum was recently revealed by R. Abbott when he found that the number of hybrid zones reported in the literature for plants is very low, given the propensity of plants to hybridise. In another literature survey on hybrid zones performed over the period 1970-2022, we found that the number of hybrid zones reported for vertebrates was 2.3 times greater than that reported for vascular plants, even though there are about six times more vascular plant species than vertebrates. Looking at the number of papers reporting hybrid zones, there are 4.9 times more on vertebrates than on vascular plants. These figures support the relevance of this conundrum. In this paper we aim to shed light on this question by providing a structured discussion of the causes that may underlie this conundrum. We propose six non-mutually exclusive factors, namely lack or deficit of spatial structure, lack or deficit of genetic structure, effects of hybridisation between non-closely related species, lability of plant hybrid zones over time, botanists' perception of hybridisation, and deficit of population genetic data. There does not appear to be a single factor that explains our puzzle, which applies to all cases of plants where hybridisation is detected but no hybrid zone is reported. It is argued that some plant features suggest that the puzzle is not, at least entirely, due to insufficient knowledge of the specific cases, a hypothesis that should be addressed with a wider range of empirical data across different taxonomic groups.

Geographic Cline and Genetic Introgression Effects on Seed Morphology Variation and Germination Fitness in Two Closely Related Pine Species in Southeast Asia

There is still limited information on how genetic introgression impacts morphological variation and population fitness in long-lived conifer species. Two closely related pine species, Pinus kesiya Royle ex Gordon and Pinus yunnanensis Franch. are widely distributed over Southeast Asia and Yunnan province of China, with a large spatial scale of asymmetric genetic introgression and hybridization, and form a hybrid lineage, P. kesiya var. langbianensis, where their ranges overlap in southeast Yunnan. We compared seed trait variation and germination performance between hybrids and parental species and characterized environmental gradients to investigate the genetic and ecological evolutionary consequences of genetic introgression. We found that seed width (SW) differed significantly among the three pines, and all the seed traits were significantly negatively correlated with latitude and associated with the mean temperatures of the driest and wettest quarters. A higher germination fitness of hybrids was detected at a low temperature, indicating that they had better adaptability to temperature stress than their parental species during the germination process. Our results suggest that environmental factors shape seed phenotypic variation in the pine species and that genetic introgression significantly affects seed germination fitness. Therefore, assisting gene flow in natural forest populations might facilitate their adaptation to climate change.

Exploring the diversity of andean berries from northern Peru based on molecular analyses

More than 12,000 species have been listed under the category of berries, and most of them belong to the orders Ericales and Rosales. Recent phylogenetic studies using molecular data have revealed disagreements with morphological approaches mainly due to diverse floral arrangements, which has proven to be a problem when recognizing species. Therefore, the use of multilocus sequence data is essential to establish robust species boundaries. Although berries are common in Andean cloud forests, diversity of these taxa has not been extensively evaluated in the current context of DNA-based techniques. In this regard, this study characterized morphologically and constructed multilocus phylogenies using four molecular markers, two chloroplast markers (matK and rbcL) and two nuclear markers (ITS and GBSSI-2). Specimens did not show diagnostic features to delimit species of berries. A total of 125 DNA-barcodes of andean berries were newly generated for the four molecular markers. The multilocus phylogenies constructed from these markers allowed the identification of 24 species grouped into the order Ericales (Cavendishia = 1, Clethra = 2, Disterigma = 2, Gaultheria = 4, Thibaudia = 4, Vaccinium = 3) and Rosales (Rubus = 8), incorporating into the Peruvian flora four new records (Disterigma ecuadorense, Disterigma synanthum, Vaccinium meridionale and Rubus glabratus) and revealing the genus Rubus as the most diverse group of berries in the Amazonas region. The results of this study showed congruence in all the multilocus phylogenies, with internal transcribed spacer (ITS) showing the best resolution to distinguish the species. These species were found in coniferous forests, dry and humid forests, rocky slopes, and grasslands at 2,506–3,019 masl from Amazonas region. The integration of morphological and DNA-based methods is recommended to understand the diversity of berries along the Peruvian Andean cloud forest.

Abstract in Quechua language

Qhawarqan astawan chunka iskayniyuq waranqa especiekuna bayasmanta huch’uy mit’a maypichus hatun rak’i chayaqi ordenkunata Ericaleswan Rosaleswan. Chayraqpi Khuski filogeneticamanta rurachiy allincharqan chanikuna molecularkuna willarqan ayñi rikunawanta morfologicokunamanta, qaylla llapan rantichay t’ika tiktutaywan ñawray, ima kay kaqta qhawacgirqan kay huk champay pachaman riqsiypa especiekunamanta. Hina kaqtintaq, chanikuna qatikipaykunamanta multilocus hat’alliy tiksipmi takyachiypaq saywakuna sinchikuna especiekunamanta. Pana bayaskuna kanku allatinkuna sach’a-sach’api phuyusqa anti runap, ñawran manan karqan achka kamaykuy kunan pacha allwiyaraykupi takyasqakuna ADN. Chayrayku, Noqanchispa taqwi allincharqan huk filogenia multilocus, rarachikupúnmi tawa molecular marcadorkuna, caspa iskay markadorkunawan cloroplastomanta (matK, rbcL) iskay markadorkunawan nuclearkunamanta (ITS, GBSSI-2). Kaykunawan filogeniamanta huniqamuran kikinchay iskay chunka tawayoq especies ima tantaqamuran q'anchis generospi (Cavendishia=1, Clethra=2, Disterigma=2, Gaultheria=4, Thibaudia=4, Vaccinium=3, Rubus=8), kaykunata huñuyqamuranta piruwanu llacha kamay tawa musuq quillqakamachikuta (Disterigma ecuadorense, Disterigma synanthum, Vaccinium meridionale, Rubus glabratus). Nocaykuq lluqsisqan kuwirinti rikuchirurqan llapankuna filogeniaspi multilocusmanta, kaspa espaciador transcrito interno (ITS) pi rikuchina kutuwi mihur rantichay riqsiypaq especiekunata.

Abstract in Awajun language

Dekanauwai juú weantug 12000 sag nagkaikiut, júna nejég tente ainawai nuintushkam kuashtai Ericales nuigtu Rosales weantui. Molecularesjai takasmaug juki filogeneticos augtus yamá dekai antugnaiñasmauwa nuna Morfologicosjai disa umikmaug, juka waignawai kuashag yagkunum, juwai dekaata tamanum kuashat utugchata ama nunuka. Nunui asamtai multilocus takasmauwa nujai dekanui wajukut ainawa pipish tumaig aidaush. Tujashkam kuashtai tentee nejég ainaug ikam naig yujagkim amuamua nunuig, wajupá kuashtakit tusajig ashi dekapasjig ADNjain dischamui. Nuni tamaugmak, ii augtusag duka takasé filogenia multilocus dekamua nujai, takasji ipák usumat marcadores molecularesjai, jimag marcadores cloroplastosjai (matK nuigtu rbcL) nuigtu jimag marcadores nuclearesjai (ITS nuigtu GBSSI-2). Juu filogenias dekaji 24 sag nagkaikiut tuwaka 7 generosnug tuwaka awa nunu (Cavendishia=1, Clethra=2, Disterigma=2, Gaultheria=4, Thibaudia=4, Vaccinium=3, Rubus=8), juui dekanai yamajam ipák usumat ajag perunum awanunu (Disterigma ecuadorense, Disterigma synanthum, Vaccinium meridionale nuigtu Rubus glabratus).

Asymmetric Reproductive Barriers and Gene Flow Promote the Rise of a Stable Hybrid Zone in the Mediterranean High Mountain

Hybrid zones have the potential to shed light on evolutionary processes driving adaptation and speciation. Secondary contact hybrid zones are particularly powerful natural systems for studying the interaction between divergent genomes to understand the mode and rate at which reproductive isolation accumulates during speciation. We have studied a total of 720 plants belonging to five populations from two Erysimum (Brassicaceae) species presenting a contact zone in the Sierra Nevada mountains (SE Spain). The plants were phenotyped in 2007 and 2017, and most of them were genotyped the first year using 10 microsatellite markers. Plants coming from natural populations were grown in a common garden to evaluate the reproductive barriers between both species by means of controlled crosses. All the plants used for the field and greenhouse study were characterized by measuring traits related to plant size and flower size. We estimated the genetic molecular variances, the genetic differentiation, and the genetic structure by means of the F-statistic and Bayesian inference. We also estimated the amount of recent gene flow between populations. We found a narrow unimodal hybrid zone where the hybrid genotypes appear to have been maintained by significant levels of a unidirectional gene flow coming from parental populations and from weak reproductive isolation between them. Hybrid plants exhibited intermediate or vigorous phenotypes depending on the analyzed trait. The phenotypic differences between the hybrid and the parental plants were highly coherent between the field and controlled cross experiments and through time. The highly coherent results obtained by combining field, experimental, and genetic data demonstrate the existence of a stable and narrow unimodal hybrid zone between Erysimum mediohispanicum and Erysimum nevadense at the high elevation of the Sierra Nevada mountains.