Isodicentric Y chromosomes and sex disorders as byproducts of homologous recombination that maintains palindromes.

Massive palindromes in the human Y chromosome harbor mirror-image gene pairs essential for spermatogenesis. During evolution, these gene pairs have been maintained by intrapalindrome, arm-to-arm recombination. The mechanism of intrapalindrome recombination and risk of harmful effects are unknown. We report 51 patients with isodicentric Y (idicY) chromosomes formed by homologous crossing over between opposing arms of palindromes on sister chromatids. These ectopic recombination events occur at nearly all Y-linked palindromes. Based on our findings, we propose that intrapalindrome sequence identity is maintained via noncrossover pathways of homologous recombination. DNA double-strand breaks that initiate these pathways can be alternatively resolved by crossing over between sister chromatids to form idicY chromosomes, with clinical consequences ranging from spermatogenic failure to sex reversal and Turner syndrome. Our observations imply that crossover and noncrossover pathways are active in nearly all Y-linked palindromes, exposing an Achilles' heel in the mechanism that preserves palindrome-borne genes.

Genetic characterization of a missense mutation in the X-linked TAF7L gene identified in an oligozoospermic man†.

Although hundreds of knockout mice show infertility as a major phenotype, the causative genic mutations of male infertility in humans remain rather limited. Here, we report the identification of a missense mutation (D136G) in the X-linked TAF7L gene as a potential cause of oligozoospermia in men. The human aspartate (D136) is evolutionally conserved across species, and its change to glycine (G) is predicted to be detrimental. Genetic complementation experiments in budding yeast demonstrate that the conserved aspartate or its analogous asparagine (N) residue in yeast TAF7 is essential for cell viability and thus its mutation to G is lethal. Although the corresponding D144G substitution in the mouse Taf7l gene does not affect male fertility, RNA-seq analyses reveal alterations in transcriptomic profiles in the Taf7l (D144G) mutant testes. These results support TAF7L mutation as a risk factor for oligozoospermia in humans.

In-vitro maturation and transplantation of cryopreserved ovary tissue: understanding ovarian longevity.

RESEARCH QUESTION: Is it possible to use experience gained from 24 years of frozen ovarian transplantation, and from recent experience with in-vitro gametogenesis to accomplish simple and robust in-vitro maturation (IVM) of oocytes from human ovarian tissue? DESIGN: A total of 119 female patients between age 2 and 35 years old underwent ovary cryopreservation (as well as in-vitro maturation of oocytes and IVM in the last 13 individuals) over a 24-year period. Up to 22 years later, 17 returned to have their ovary tissue thawed and transplanted back. RESULTS: Every woman had a return of ovarian function 5 months after transplant, similar to previous observations. As observed before, anti-Müllerian hormone (AMH) concentration rose as FSH fell 4 months later. The grafts continued to work up to 8 years. Of the 17, 13 (76%) became pregnant with intercourse at least once, resulting in 19 healthy live births, including six live births from three women who had had leukaemia. Of the harvested germinal vesicle oocytes, 35% developed with simple culture media into mature metaphase II oocytes. CONCLUSIONS: The authors concluded the following. First, ovary tissue cryopreservation is a robust method for preserving fertility even for women with leukaemia, without a need to delay cancer treatment. Second, many mature oocytes can often be obtained from ovary tissue with simple media and no need for ovarian stimulation. Third, ovarian stimulation only be necessary for removing the oocyte from the ovary, which can also be accomplished by simple dissection at the time of ovary freezing. Finally, pressure and just eight 'core genes' control primordial follicle recruitment and development.

Unraveling transcriptome dynamics in human spermatogenesis.

Spermatogenesis is a dynamic developmental process that includes stem cell proliferation and differentiation, meiotic cell divisions and extreme chromatin condensation. Although studied in mice, the molecular control of human spermatogenesis is largely unknown. Here, we developed a protocol that enables next-generation sequencing of RNA obtained from pools of 500 individually laser-capture microdissected cells of specific germ cell subtypes from fixed human testis samples. Transcriptomic analyses of these successive germ cell subtypes reveals dynamic transcription of over 4000 genes during human spermatogenesis. At the same time, many of the genes encoding for well-established meiotic and post-meiotic proteins are already present in the pre-meiotic phase. Furthermore, we found significant cell type-specific expression of post-transcriptional regulators, including expression of 110 RNA-binding proteins and 137 long non-coding RNAs, most of them previously not linked to spermatogenesis. Together, these data suggest that the transcriptome of precursor cells already contains the genes necessary for cellular differentiation and that timely translation controlled by post-transcriptional regulators is crucial for normal development. These established transcriptomes provide a reference catalog for further detailed studies on human spermatogenesis and spermatogenic failure.

When "facts" are not facts: what does p value really mean, and how does it deceive us?

The recent paper in JAMA alleging that frozen embryo transfer causes twice the risk of childhood cancer in the offspring is an excellent example of the erroneous use of statistical tests (and the misinterpretation of p value) that is common in much of the medical literature, even in very high impact journals. These myths backed by misleading statements of "statistical significance" can cause far-reaching harm to patients and doctors who might not understand the pitfalls of specious statistical testing.

Success rates in minimal stimulation cycle IVF with clomiphene citrate only.

PURPOSE: To determine age-adjusted overall success rates for patients undergoing clomiphene citrate only minimal stimulation cycle (mini) in vitro fertilization (IVF) without any gonadotropin administration. METHODS: Eight hundred thirty-nine women (mean age: 38.4 ± 0.1 years; 2488 cycles) underwent clomiphene citrate only mini-IVF. Their first oocyte retrieval was between January 2009 and December 2009, with follow-up until December 2014. The cumulative live birth rate (CLBR) per oocyte retrieval cycle started and live birth rate per oocyte was retrospectively analyzed. The basic CLBR was calculated as the number of women who achieved a live birth divided by the total number of women who started oocyte retrieval. RESULTS: The mean number of oocytes retrieved was 1.5. The basic CLBRs for all ages after the first and third cycles were 22.6% and 39.2%, respectively. For ≤ 34 years, 35-37 years, 38-40 years, 41-42 years, and ≥ 43 years, CLBRs after the first and third cycles were 42.5% and 70.1%, 32.9% and 49.1%, 20.0% and 38.6%, 12.6% and 25.2%, and 4.4% and 8.8%, respectively. These rates had a significant relationship with age (P < 0.01). The LBR per oocyte for all ages was 9.6%. CONCLUSION: Acceptable overall IVF success rates can be achieved in clomiphene citrate only mini-IVF, as well as acceptable LBR. The CLBRs and LBRs per oocyte are evidently influenced by women's age.

Apoptosis of mural granulosa cells is increased in women with diminished ovarian reserve.

PURPOSE: To evaluate the relationship between apoptosis of granulosa cells in women with normal ovarian reserve versus diminished ovarian reserve, and relate that to follicular fluid hormones, and to clinical outcomes. METHODS: A prospective cohort study was initiated between October 2015 and June 2016 involving a total of 164 women undergoing IVF/ICSI cycles at a single IVF center. Mural and cumulus granulosa cells, and follicularfluid were collected during oocyte retrieval. Annexin V-FITC/PI apoptosis staining and flow cytometryanalysis were performed to evaluate apoptosis rate of mural granulosa cells and cumulus cells. Follicularfluid hormones were measured by ECLIA. Laboratory and clinical outcomes were analyzed. RESULTS: In mural granulosa cells, early, late and total apoptosis rates were significantly increased in women with diminished ovarian reserve when compare to women with normal ovarian reserve, along with lower AMHand progesterone levels (but higher estradiol levels) in follicular fluid. Early apoptosis rate of cumulus cellswas significantly higher in the non-pregnant group. The apoptosis rate of mural cells was negativelycorrelated with parameters related to ovarian response, oocyte yield, MII egg number, 2pn cleavagenumber, D3 good embryos number, blastocyst formation rate and frozen embryos number. A positivecorrelation was found between mural granulosa cell apoptosis and age. CONCLUSION: A significantly higher apoptosis rate of mural granulosa cells was correlated with worse ovarian response, with fewer egg and embryo numbers in IVF/ICSI, as well as with age. Early apoptosis rate of cumulus cellsmight also have influence on clinical pregnancy.

The varicocele argument resurfaces.

A recent series of articles and reviews published in Fertility and Sterility have rekindled the more than half century debate on varicocelectomy. Every one of these articles favored strongly the repair of varicocele for male infertility. Since my review paper on this issue in 2001, published in Human Reproduction Update, and since advent of ICSI in 1993, I had thought that most reproductive physicians felt negatively about the benefit of varicocelectomy. However, more recent urological papers are causing this negative view to be re-evaluated. It is now advocated by some urologists that varicocelectomy improves sperm count and testosterone levels, and even improves the results with ICSI. Thus, it may be appropriate to revisit older studies again and review the newer ones in this never ending controversy. Newer studies are re-opening the door to review and possibly re-instate varicocelectomy. This dilemma may never be fully resolved, but it is important to keep an open mind.

Cryopreservation and transplantation of ovarian tissue: results from one center in the USA.

PURPOSE: To report the results of cryopreserved ovary tissue transplantation for leukemia and other cancers, in a single US center. METHODS: One hundred eight females between age 6 and (median age 24) 35 were referred for possible ovary tissue cryopreservation over a 20-year period, with either slow freeze or vitrification. Thus far 13 patients returned up to 18 years later to have their tissue transplanted back. RESULTS: All 13 patients had return of ovarian function 5 months post transplant with regular menstrual cycling. AMH rose to very high levels as the FSH declined to normal. Four months later, the AMH again declined to very low levels. Nonetheless, the grafts remained functional for up to 5 years or longer. Ten of the 13 (77%) became spontaneously pregnant at least once, resulting in 13 healthy babies. A total of 24 healthy babies have been born 11 from fresh transplanted ovarian tissue and 13 from cryopreserved transplanted ovarian tissue. CONCLUSIONS: (1) Ovary tissue cryopreservation is a robust method for preserving a woman's fertility. (2) Cortical tissue pressure may be a key regulator of primordial follicle arrest, recruitment, and ovarian longevity. (3) This is the only such series yet reported in the USA.

Live birth derived from oocyte spindle transfer to prevent mitochondrial disease.

Mutations in mitochondrial DNA (mtDNA) are maternally inherited and can cause fatal or debilitating mitochondrial disorders. The severity of clinical symptoms is often associated with the level of mtDNA mutation load or degree of heteroplasmy. Current clinical options to prevent transmission of mtDNA mutations to offspring are limited. Experimental spindle transfer in metaphase II oocytes, also called mitochondrial replacement therapy, is a novel technology for preventing mtDNA transmission from oocytes to pre-implantation embryos. Here, we report a female carrier of Leigh syndrome (mtDNA mutation 8993T > G), with a long history of multiple undiagnosed pregnancy losses and deaths of offspring as a result of this disease, who underwent IVF after reconstitution of her oocytes by spindle transfer into the cytoplasm of enucleated donor oocytes. A male euploid blastocyst wasobtained from the reconstituted oocytes, which had only a 5.7% mtDNA mutation load. Transfer of the embryo resulted in a pregnancy with delivery of a boy with neonatal mtDNA mutation load of 2.36-9.23% in his tested tissues. The boy is currently healthy at 7 months of age, although long-term follow-up of the child's longitudinal development remains crucial.

Ovarian tissue cryopreservation and transplantation: scientific implications.

After fresh or frozen ovary transplantation, FSH levels return to normal, and menstrual cycles resume by 150 days, coincident with anti-Müllerian hormone rising to higher than normal levels. AMH then returns to well below normal levels by 240 days, remaining as such for many years with nonetheless normal ovulation and fertility. To date, 20 babies have been born in our program from 11 fresh and 13 cryopreserved ovary transplant recipients with a live baby rate of over 70 % (11 babies from fresh and 9 from frozen). Globally, over 70 live births have been reported for both fresh and frozen ovary transplants with an approximate 30 % live birth rate. Given the rapid rise of AMH after the fall of FSH, with a subsequent AMH decrease with retention of ovarian function, it is tempting to speculate the existence of a shared mechanism controlling primordial follicle recruitment, fetal oocyte meiotic arrest, and recruitment in the adult ovary. With the massive recruitment of primordial follicles observed after human ovarian cortical tissue transplantation, which subsides to an extremely low recruitment rate, we will discuss how this phenomenon suggests a unifying theory implicating ovarian cortical tissue rigidity in the regulation of both fetal oocyte arrest and recruitment of follicles in the adult ovary. As the paper by Winkler-Crepaz et al. in this issue demonstrates, our in vivo results are consistent with the in vitro demonstration that primordial follicles in the fetal cortex are "locked" in development, resulting in meiotic arrest, which spares the oocytes from being rapidly lost all at once (Winkler-Crepaz et al., J Assist Reprod Genet, 1). Winkler-Crepaz et al. demonstrate that follicle loss after ovarian cortex transplantation is unlikely due to ischemic apoptosis, but rather from a "burst" of primordial follicle recruitment. In vivo, primordial follicles are normally resistant to further development or activation to prevent oocyte depletion. The dense fibrous ovarian cortex, through as yet unresolved mechanisms, arrests the further continuation of meiosis and also prevents a sudden depletion of all resting follicles in the adult ovary. Intrinsic tissue pressure is released after cortical tissue transplantation, temporarily resulting in a rapid follicle depletion. These results are consistent with the observation that once the ovarian reserve is reduced in the graft, the rate of recruitment diminishes and the ovarian tissue exhibits a relatively long duration of function.

AZFc deletions and spermatogenic failure: a population-based survey of 20,000 Y chromosomes.

Deletions involving the Y chromosome's AZFc region are the most common known genetic cause of severe spermatogenic failure (SSF). Six recurrent interstitial deletions affecting the region have been reported, but their population genetics are largely unexplored. We assessed the deletions' prevalence in 20,884 men in five populations and found four of the six deletions (presented here in descending order of prevalence): gr/gr, b2/b3, b1/b3, and b2/b4. One of every 27 men carried one of these four deletions. The 1.6 Mb gr/gr deletion, found in one of every 41 men, almost doubles the risk of SSF and accounts for ∼2% of SSF, although <2% of men with the deletion are affected. The 1.8 Mb b2/b3 deletion, found in one of every 90 men, does not appear to be a risk factor for SSF. The 1.6 Mb b1/b3 deletion, found in one of every 994 men, appears to increase the risk of SSF by a factor of 2.5, although <2% of men with the deletion are affected, and it accounts for only 0.15% of SSF. The 3.5 Mb b2/b4 deletion, found in one of every 2,320 men, increases the risk of SSF 145 times and accounts for ∼6% of SSF; the observed prevalence should approximate the rate at which the deletion arises anew in each generation. We conclude that a single rare variant of major effect (the b2/b4 deletion) and a single common variant of modest effect (the gr/gr deletion) are largely responsible for the AZFc region's contribution to SSF in the population.

Fertility preservation for age-related fertility decline.

Cryopreservation of eggs or ovarian tissue to preserve fertility for patients with cancer has been studied since 1994 with R G Gosden's paper describing restoration of fertility in oophorectomised sheep, and for decades previously by others in smaller mammals. Clinically this approach has shown great success. Many healthy children have been born from eggs cryopreserved with the Kuwayama egg vitrification technique for non-medical (social) indications, but until now very few patients with cancer have achieved pregnancy with cryopreserved eggs. Often, oncologists do not wish to delay cancer treatment while the patient goes through multiple ovarian stimulation cycles to retrieve eggs, and the patient can only start using the oocytes after full recovery from cancer. Ovarian stimulation and egg retrieval is not a barrier for patients without cancer who wish to delay childbearing, which makes oocyte cryopreservation increasingly popular to overcome an age-related decline in fertility. Cryopreservation of ovarian tissue is an option if egg cryopreservation is ruled out. More than 35 babies have been born so far with cryopreserved ovarian tissue in patients with cancer who have had a complete return of hormonal function, and fertility to baseline. Both egg and ovarian tissue cryopreservation might be ready for application to the preservation of fertility not only in patients with cancer but also in countering the increasing incidence of age-related decline in female fertility.

Live birth rates after MESA or TESE in men with obstructive azoospermia: is there a difference?

STUDY QUESTION: How do live birth rates compare after intracytoplasmic sperm injection (ICSI) for men with obstructive azoospermia when using sperm derived from testicular sperm extraction (TESE) versus microsurgical epididymal sperm aspiration (MESA)? SUMMARY ANSWER: Our study suggests that proximal epididymal sperm (from MESA) result in higher live birth rates as compared with testicular sperm (from TESE) in couples where the man has obstructive azoospermia due to congenital bilateral absence of the vas deferens (CBAVD) or vasectomy. WHAT IS KNOWN ALREADY: For couples with obstructive azoospermia, MESA (epididymal sperm) and TESE (testicular sperm) have generally been assumed to be equivalent for use in ICSI. But this assumption has never been confirmed, and this view has important clinical and basic scientific consequences. STUDY DESIGN, SIZE, DURATION: This was a retrospective study of a consecutive cohort of 374 men with obstructive azoospermia and normal spermatogenesis, who underwent IVF and ICSI using either epididymal sperm or testicular sperm in the period 2000-2009. PARTICIPANTS/MATERIALS, SETTING, METHODS: The study included men undergoing MESA or TESE at St. Luke's Hospital for obstructive azoospermia due to CBAVD or vasectomy. MAIN RESULTS AND THE ROLE OF CHANCE: A total of 280 couples underwent MESA and 94 underwent TESE with ICSI. The live birth rate was 39% after MESA-ICSI and 24% after TESE-ICSI. The MESA-ICSI cycles also resulted in a significantly higher implantation rate and significantly higher clinical and ongoing pregnancy rates than the TESE-ICSI cycles. There was no significant difference in results between fresh or frozen sperm for both MESA and TESE. When adjusted for the available confounders, the odds ratio for live birth was significantly in favour of MESA-ICSI versus TESE-ICSI (OR 1.82; 95% CI 1.05-3.67). The only significant confounders were female age and ovarian reserve. LIMITATIONS, REASONS FOR CAUTION: This is a retrospective cohort study and not a randomized clinical trial. WIDER IMPLICATIONS OF THE FINDINGS: Our study suggests that some aspect of sperm maturation after the sperm leaves the testicle to enter the epididymis is required for the most optimal results, even when ICSI is used for fertilization. STUDY FUNDING/COMPETING INTERESTS: No funding was used and there are no competing interests.

Unifying theory of adult resting follicle recruitment and fetal oocyte arrest.

One of the biggest mysteries of ovarian physiology is what controls the emergence of adult primordial follicles from the resting stage, and their steady depletion over the woman's lifetime. A related mystery is why do early oogonia begin meiosis in the fetus and then suddenly arrest for most of fetal and adult life. If fetal oocyte arrest did not occur after meiotic activation, there would be no oocytes left in the female baby by the time she is born. Similarly, without a steady controlled release in the adult ovary of resting follicles, the adult woman would run out of her eggs prematurely and have an early menopause. Could there be a similarity between what causes fetal oocyte arrest and what causes adult oocyte recruitment? The answer begins with the observation of a sudden massive recruitment of primordial follicles after human ovarian transplantation, and the embryologic discoveries about oocyte activation and the time of differentiation of cortex and medulla. The unifying theory is that ovarian cortical tissue pressure controls both fetal oocyte arrest and adult oocyte recruitment.

Fresh and cryopreserved ovary transplantation and resting follicle recruitment.

Ovary cryopreservation and transplantation has garnered increasing interest as a possible method to preserve fertility for cancer patients and to study ovarian resting follicle recruitment. Eleven consecutive women underwent fresh donor ovary transplantation, and 11 underwent cryopreserved ovary auto-transplantation in the same centre, with the same surgeon. Of the 11 fresh transplant recipients, who were all young but menopausal, nine women had normal ovarian cortex transplanted from an identical twin sister, and two had a fresh allograft from a non-identical sister. In the second group, 11 women with cancer had ovarian tissue cryopreserved before bone marrow transplant, and then after years of therapeutically induced menopause, underwent cryopreserved ovarian cortex autotransplantation. Recovery of ovarian function and follicle recruitment was assessed in all 22 recipients, and the potential for pregnancy was further investigated in 19 (11 fresh and 8 cryopreserved) with over 1-year follow-up. In all recipients, normal FSH levels and menstruation returned by about 150 days, and anti-Müllerian hormone reached much greater than normal concentrations by about 170 days. Anti-Müllerian hormone levels then fell below normal by about 240 days and remained at that lower level. Seventeen babies have been born to these 11 fresh and eight cryopreserved ovary transplant recipients.

In Vitro Maturation, In Vitro Oogenesis, and Ovarian Longevity.

This paper will review a remarkable new approach to in vitro maturation "IVM" of oocytes from ovarian tissue, based on our results with in vitro oogenesis from somatic cells. As an aside benefit we also have derived a better understanding of ovarian longevity from ovary transplant. We have found that primordial follicle recruitment is triggered by tissue pressure gradients. Increased pressure holds the follicle in meiotic arrest and prevents recruitment. Therefore recruitment occurs first in the least dense inner tissue of the cortico-medullary junction. Many oocytes can be obtained from human ovarian tissue and mature to metaphase 2 in vitro with no need for ovarian stimulation. Ovarian stimulation may only be necessary for removing the oocyte from the ovary, but this can also be accomplished by simple dissection at the time of ovary tissue cryopreservation. By using surgical dissection of the removed ovary, rather than a needle stick, we can obtain many oocytes from very small follicles not visible with ultrasound. A clearer understanding of ovarian function has come from in vitro oogenesis experiments, and that explains why IVM has now become so simple and robust. Tissue pressure (and just a few "core genes" in the mouse) direct primordial follicle recruitment and development to mature oocyte, and therefore also control ovarian longevity. There are three distinct phases to oocyte development both in vitro and in vivo: in vitro differentiation "IVD" which is not gonadotropin sensitive (the longest phase), in vitro gonadotropin sensitivity "IVG" which is the phase of gonadotropin stimulation to prepare for meiotic competence, and IVM to metaphase II. On any given day 35% of GVs in ovarian tissue have already undergone "IVD" and "IVG" in vivo, and therefore are ready for IVM.

Derivation of human primordial germ cell-like cells in an embryonic-like culture.

Primordial germ cells (PGCs) are the embryonic precursors of sperm and eggs. They transmit genetic and epigenetic information across generations. Given the prominent role of germline defects in diseases such as infertility, detailed understanding of human PGC (hPGC) development has important implications in reproductive medicine and studying human evolution. Yet, hPGC specification remains an elusive process. Here, we report the induction of hPGC-like cells (hPGCLCs) in a bioengineered human pluripotent stem cell (hPSC) culture that mimics peri-implantation human development. In this culture, amniotic ectoderm-like cells (AMLCs), derived from hPSCs, induce hPGCLC specification from hPSCs through paracrine signaling downstream of ISL1. Our data further show functional roles of NODAL, WNT, and BMP signaling in hPGCLC induction. hPGCLCs are successfully derived from eight non-obstructive azoospermia (NOA) participant-derived hPSC lines using this biomimetic platform, demonstrating its promise for screening applications.

Oophorectomy for fertility preservation via reduced-port laparoscopic surgery.

BACKGROUND: For fertility preservation of women patients scheduled to undergo chemotherapy or radiotherapy, unilateral oophorectomy was performed, and the ovary was cryopreserved. METHODS: Two-port surgery was conducted in 3 patients, and single-port surgery using a single-incision laparoscopic surgery port in 3. An 18-G Cathelin needle equipped with a syringe was directly inserted transabdominally to reach the small follicle on the ovarian surface; then, follicular fluid was recovered by aspiration through the syringe as with in vitro fertilization procedures, and immature oocytes were collected from the resulting culture medium under microscopy and cryopreserved. Vitrification of the ovarian tissue was performed using the cryotissue method. RESULTS: The operative time and estimated blood loss were 39.7 minutes (17-57) and 8.6 mL (2-20), and the numbers of ovarian cortical tissues and immature oocytes collected were 10.1 (5.5-15) and 16.3 (0-36), respectively. CONCLUSIONS: It is suggested that fertility preservation operations before chemotherapy or radiotherapy can be safely done using reduced-port surgery.

Polymorphism for a 1.6-Mb deletion of the human Y chromosome persists through balance between recurrent mutation and haploid selection.

Many human Y-chromosomal deletions are thought to severely impair reproductive fitness, which precludes their transmission to the next generation and thus ensures their rarity in the population. Here we report a 1.6-Mb deletion that persists over generations and is sufficiently common to be considered a polymorphism. We hypothesized that this deletion might affect spermatogenesis because it removes almost half of the Y chromosome's AZFc region, a gene-rich segment that is critical for sperm production. An association study established that this deletion, called gr/gr, is a significant risk factor for spermatogenic failure. The gr/gr deletion has far lower penetrance with respect to spermatogenic failure than previously characterized Y-chromosomal deletions; it is often transmitted from father to son. By studying the distribution of gr/gr-deleted chromosomes across the branches of the Y chromosome's genealogical tree, we determined that this deletion arose independently at least 14 times in human history. We suggest that the existence of this deletion as a polymorphism reflects a balance between haploid selection, which culls gr/gr-deleted Y chromosomes from the population, and homologous recombination, which continues to generate new gr/gr deletions.