The 7 Sexes: Biology of Sex Determination

1. Introduction

1 Introduction With rare exceptions, animals consist of sexually reproducing populationsthatareroughlyhalfmaleandhalffemale —atleastthatisahuman perspective that is applied to other mammals, and generalized to all other animals. An observant individual will notice roaches mating rear end to rear end or horseshoe crabs on the beach in springtime mating with the male mounted on a female, reinforcing the idea that the image of human intercourse can be generalized. I can observe fruit flies mating in the same way without use of a microscope, and I can even tell which is male and which is female if I am looking at a solitary fruit fly resting on my finger. But that idea of universality is undermined if I observe copulating earthworms,whichseemtobeengagedinsomesortofsymmetricalmutual engagement. The ambiguity of the earthworm’s hermaphroditism is also present in most flowering plants. Students learn that pollen bearing stamens are present in the same flower with female components—assigned scholarly names like stigma, style, and ovary—but that is also not universal. Until the invention of microscopes in the 1660s, the world of the very small organisms—or parts of organisms, like cells—was closed off to human observation; almost all early ideas of sex determination are rooted in what could be seen with the unassisted eye. The “two by two” image of sexuality is reinforced in the story of Noah’s ark, but the story of sex determination in the Bible is puzzling to a reflective reader. Adam, clearlyafunctional male atcreation,is givenacompanionoutof hisown rib and she is called Eve, but the special creation of the female is necessary only in the human species. Water, air, and land animals are created inGenesisequallyasmaleandfemale(oneassumes)inanindeterminate 2 numberwithinstructionstobefruitfulandmultiply;thereisno“second sex” creation for the rest of life. This book is a history of our ideas about sex determination—from ancient myths to present day molecular insights. I got interested in the history of human sexuality after teaching a course on the biology of human reproduction at Stony Brook University. Sometime around 1990, Ruth Cowan asked me if I knew anyone who could teach such a course foranundergraduateprogramshewasdeveloping.Ivolunteeredtoteach the course myself and spent a summer at the medical school library preparing the lectures and a text (unpublished) for it. The course was well received and I continued to teach it until I retired in 2001. When there was no science, as we know it, speculation associated with religious writings prevailed on the determination of sex: Judaism, Christianity,andIslamdrewtheirinterpretationsfromthebookofGenesis . Other religious traditions abound, which folklore scholars have studied using appropriate motifs of gender, sexual reproduction, twinning , hermaphroditism, and other variations. In the western tradition, the first scholarly attempts at studying sex determination are associated with Greek philosophers, but their insights are not very helpful because the tools of science were absent. Humans are adept at framing stories that explain complex reality as best as they can, but observation alone provides limited information. The turning point in the history of sex determination arose with the introduction of the microscope when, for the first time, microscopic anatomy was revealed. It then took almost two centuries after cells were first named for the cell theory to emerge. Almost all of our present day knowledge of the biology of sex determination is relatively recent, primarily worked out in the twentieth century. For the duration of written history prior to the twentiethcentury,sex determinationwas embedded inreligioustraditions,taboos,andmoraltransgressions.Severepenalties couldbeimposedifthoseviolatingthesenormswereidentifiedandtried by religious or secular courts. Each component of human sexuality has its own history. The story of sperm, eggs, gonads, external genitals, internal genitals, sex in different stages of the life cycle, pregnancy, twinning, hormonal regulation, fertilization, alternation of haploid and diploid cellular states, the role Introduction 3 of meiosis, sex chromosomes, and many other features of the complex events and developmental anatomy of reproduction was worked out and by the 1960s a fairly modern understanding was available. All that was lacking was a molecular interpretation of the way sex determination works, and that too yielded to studies in the last third of the twentieth century. In addition to the normal sequence of events leading to functional malesandfunctionalfemalescopulatingasheterosexualcouples,anumber of variations existed. Some involved rare ambiguities of genitals called, at the time, hermaphroditism, chimerism, or pseudohermaphroditism . Some involved equally rare disturbances of sexual development associated with sex chromosome aneuploidy. Some involved possible genetic or gestational conditions that led to changes in sexual orientation including homosexuality, which is especially of interest to society because of its...