Lange, Natalie Mother’s Genetics Create an Environmental Tension for Her Infant
Faculty Mentor: James Dee Higley, Ph.D., Department of Psychology
Over the summer of 2016 the generous donations for the ORCA funds were utilized to investigate the influence of maternal serotonin transporter genotype (5-HTT) on infant temperament. This gave us the potential to analyze infant behavioral data for evidence of mother-infant attachment quality and possible conflict in the mother-infant relationship. The code that makes up a gene is a double based pair, with one of the bases coming from mother and a second from the father which may be identical or different. The 5-HTTLPR genotype has two allele variations: a short (s) and long (L) allele variant. The short allele of 5-HTTLPR has been associated with depression, anxiety, aggressive behavior and M-I attachment quality in both humans and rhesus macaques (Caspi, 2010; Spinelli, 2012). However, some research points to the s allele as conferring more attentive care by mothers to their infants, resulting in higher attachment quality (Mileva-Seitz, 2011). Infants with the s allele, on the other hand, exhibit less quality interactions with their mother (Spinelli, 2012). The initial hypotheses of this ORCA grant were that when maternal genotype and infant genotype are mismatched (e.g., mother genotype is Ls or ss while infant genotype is LL), the resulting maternal treatment would cause strain and conflict within the M-I dyad. Compared to mother-infant pairs that are matched in genotype, mismatched pairs were expected to show extremes in infant temperament and increased M-I conflict. However, preliminary data from the California Primate Center (hereafter CNPRC), found in the BioBehavioral Assessment (BBA) database were used prior to going to Davis this summer to complete behavioral observations. These preliminary analyses on the relationship between matched and mismatched mother-infant genotypes in regard to temperament scores yielded nonsignificant results. In light of these results from the pilot data we modified the hypotheses for the project. We decided to make the focal subject of the behavioral observations the mothers, as many of the previous studies focused either wholly on the behaviors of the infant or on the dyad, leaving on void in the research because we do not have any data that focuses specifically on maternal behaviors. Since the CNPRC has such a large population of rhesus macaques our lab had the rare privilege to observe mothers homozygous for the s allele. Most papers investigating the serotonin transporter gene, have only been able to investigate mothers possessing one copy of the s allele because the ss genotype subjects are either culled or die. Having this population to study is an opportunity to shed some more light on temperamental outcomes of infants of mothers who possess two copies of the s allele is important because it may explain human mother infant relationships. For this project we took advantage of this opportunity to compare how mother homozygous for the s allele were potentially different in their mothering behaviors from their LL or Ls counterparts. In recent literature there have been mixed results as to if mothers possessing only a single copy of the s allele of the 5-HTT genotype grants mothers an advantage or stumbling block in regard to how they treat their infants. The data gathered this summer will hopefully contribute to this growing body of research with our new hypotheses: If a mother is homozygous for the short allele we expect that they will demonstrate higher frequencies of behaviors either associated with overprotectiveness or overly laissez-faire mothering styles when compared to others. Mothering styles such as these may be a
significant factor in the development of infant psychopathology later in life.
To better understand the complex relationship between mothers and infants, a nonhuman primate species similar to humans was used as a model for study. The rhesus macaque (Macaca mulatta) is a commonly used nonhuman primate species used in research regarding temperament development as rhesus monkey infants share similar temperament dimensions with human infants (Kay et al., 2010). In addition to the rhesus infants being a model for infant temperament development, rhesus macaques have complex social structures in their colonies, use a multiplex of body language and vocalizations to communicate, and are fully dependent on their mothers at birth allowing this species of primate to be an ideal organism to study for mother-infant interactions. Whether in rhesus macaques or humans, the 5-HTT gene has a profound influence on temperament (In humans: Bouvette-Turcot et al., 2015; In rhesus macaques: McCormack, Newman, Higley, Maestripieri, & Sanchez, 2009; Suomi et al., 2011). To study the influence of a mother’s 5-HTT genotype on her infant’s temperament 18 mother-infant pairs were selected from the CNPRC in Davis, California for observation over the course of the summer.
The CNPRC has a population of approximately 5000 rhesus macaques, of which Dr. Higley’s lab has measured temperament in 626 (317 males, 309 females) infants using the BBA and we obtained 5-HTT genotype from the Primate Center. This research performed by the CNPRC was used to select female rhesus macaques who had given birth last year for observational study. The selection criteria for subjects of the observational studies were that the female had to have given birth in the spring of 2016, mother and infant 5-HTT genotype, that the mother had been assessed for her temperament when she was an infant. While the original proposal hoped to observe 50 mother-infant pairs, there were only 18 mother-infant pairs who fit the overall selection criteria needed for behavioral observations. Of the three 5-HTT genotypes possible for the mothers to have (LL, Ls, ss) the lab selected subjects of each maternal genotype to observe with their infants. An ethogram was created based on previous research dictating which maternal behaviors and interactions between herself, her infant, and other individuals would be recorded for further analysis. These behaviors included, but were not limited to, mother-infant proximity, frequency of time spent on the mother’s ventrum, rejections, approaches, leaves, and acts of aggression.
We will analyze this data next semester using ANOVA, with the independent variables being mother and infant genotypes, while the dependent variable will be the frequencies of the mother-infant behaviors observed, with an emphasis on maternal genotype (particularly the homozygous s-allele mothers) influences on the mother’s treatment of her infant.
While our timeline for this project has been pushed back, the overall goal of using this data collected over the summer is still to be presented at the Mary Lou Fulton Research Symposium at BYU this winter (2017), will be submitted for the poster session at the conference of the American Society of Primatology this coming fall, and will hopefully culminate in an article for publication. More data collection will also be performed over the coming summer internship at the CNPRC to increase the sample size.