Changing of reproductive mode, a balanced affair between genetics and environment – SexAphid

For living organisms, it is essential to adapt to their environment, both on the short term to allow an individual to survive, and on the longer term to assure species perpetuation. Adaptation requires flexibility: at the scale of an individual, this means to adapt genetic programs to the changing local environment, and on the long term, to produce the right genetic combinations on which selection will act. Sexual reproduction is the most frequent mode in metazoans and confers high level of genetic diversity and evolvability. Some species are known to be asexual, and the molecular mechanisms underlying this trait and creating genetic diversity are still largely unknown. Aphids are insect pests that are capable of both reproductive modes; an asexual viviparous reproduction, clonal, rapid and efficient that allows a proliferation of aphid populations during spring and summer, and an oviparous sexual reproduction, in autumn, that allows genetic combinations. The eggs made from this sexual mode are cold-resistant and allow survival during hard winters. These populations that alternate of reproductive modes are called “CP” (Cyclical Parthenogenesis). This capacity of aphids to alternate between reproductive modes is however sometimes lost in some aphid populations: as a consequence, such populations are no more able to produce eggs and might disappear if winter temperatures are low; such populations are called “OP” (“Obligate Parthenogenesis”). At that stage, climate plays a key role on the distribution between those two types of populations: populations still able to produce sexual individuals will have an advantage under continental climates, whereas populations that have lost the capacity to produce sexual individuals will have an advantage under warmer climates. For temperate climates, both types of populations can co-exist, with different occurrences from one year to another, depending on the winter temperatures.
SexAphid aims at i) characterizing and identifying the loci and gene functions that differentiate sexual and asexual populations, ii) assessing the frequency distribution of sexual and asexual populations in the last 15 years, and iii) identifying climatic periods, in particular winter temperatures, which best explain these distributions. SexAphid focuses on the pea aphid Acyrthosiphon pisum for which a lot of genomics and genetics data and tools are already available. Those tools and data will be used to identify the polymorphic zones of the pea aphid genome that control reproductive mode. A previous work already identified a genomic zone covering 10 cM of the X chromosome as being very linked to the CP/OP phenotype. We aim to restrict this zone and to define the genomic loci that are tightly linked with the phenotype of interest, and to point out candidate genes. The functions of those candidate genes will be deeply studied by adapted spatio-temporal expression analyses in different tissues, developmental stages and sexual and/or asexual populations. Genome editing techniques such as CRISPR-CAS9 will allow testing the precise roles of some of the candidate genes in the control of OP/CP phenotype. These polymorphic characterizations will be used to DNA-type and track wild populations on the pea aphid, captured during the last 15 years, and archived in the lab, in order to test the presence of correlations between sexual/asexual population frequencies and winter temperatures.
As aphids are pest for crops, this work should pave the road for the future development of tools to predict and prevent aphid infestation, in order to improve insect pest management. Our hypothesis is that winter temperature increase in a temperate climate might favor the development of asexual populations and thus increase the damage caused by aphids to crops. SexAphid combines genetics, genomics and ecological approaches, with cross-talked between the expertise of the 3 different academic partners, for an ANR budget of approx. 380 keuros.